Climate Science

How Do We Know What We Know About Climate Change?

8/20/2010

By Walt Meier, The National Snow and Ice Data Center  
 

It has been has been an eventful summer so far in terms of weather and climate. We’ve had strong heat waves on the U.S. east coast, and even more searing heat in parts of Russia. Along with the Russian heat wave have come enormous fires near Moscow and in Siberia. In the Arctic, we’ve seen a large chunk of ice, more than four times the size of Manhattan, break off the Petermann Glacier in northwestern Greenland. Also, we’re seeing yet another very low year for Arctic sea ice, particularly in the fabled Northwest Passage where ice has already substantially cleared out much earlier in the summer, more than ever seen before. While it is doubtful we will reach the record-low sea ice levels seen in 2007, we have already passed what used to be normal conditions with about a month of melt left to go.

As discussed in my previous entry, events such as heat waves, fires, iceberg calvings, and even low sea ice summers have occurred in the past and can be attributed to weather, and thus are not in and of themselves conclusive evidence of global warming. Similarly the big snowstorms in Washington, DC and the cold weather in Europe were not evidence against global warming. Nevertheless, more frequent and harsher extreme weather events (even snow storms) are expected in a warming world. So, while these extreme conditions are not conclusive, they do fit a pattern we expect to see from warming. In this post, I want to step back a bit and discuss the conclusive evidence that we do have and how we’ve obtained that evidence.

First, we have the thermometer records from weather stations. There are many records dating back to the mid-1800s, giving us over 150 years of temperature data. There are also ship records of sea surface temperature dating back many years. Balloons have been used to regularly sample the air throughout the atmosphere since the 1950s. More recently, satellites have expanded our data collecting potential exponentially. Some satellite records go back to the early 1970s, giving us over 30 years of data – long enough to confidently track long-term climate trends. These satellite data records include: air/land/sea surface temperature, sea ice, glaciers, ice sheets, sea level, vegetation, ocean color (biological productivity in the oceans), and others.

Aircraft are used to fill in measurements between the local in situ measurements and the large-scale satellite measurements. Submarines have been used to sample ocean conditions beneath the sea surface as well as measure the thickness of sea ice from below.

Carbon dioxide measurements taken on Mauna Loa in Hawaii since 1958 provide us with clear evidence of the effect of human emissions on the atmosphere. Carbon dioxide is now routinely measured at dozens of sites around the world.

Sometimes even something as simple as a photo can be global warming data. Photographs of glaciers taken several decades ago can be compared to new photos taken from the same location to see how glaciers have retreated. Likewise, landscape photos in the Arctic show how the tundra is becoming “shrubbified” with greener, leafier plants replacing the tundra. Even a lottery can be global warming evidence. For 94 years now people in Fairbanks, Alaska have been placing bets on when the Nenana River, which flows through town, will break up each spring. As a result we have nearly a century-long record of warming spring conditions in Fairbanks.

Still, human measurements, no matter how unconventional, cover only a relatively short period of earth’s climate history.  Thus our observations are supplemented by paleoclimate “proxy” measurements – not direct measurements of, say, temperature, but of something else that varies with temperature. Examples are tree ring data, which provide a temperature record over the last ~1000 years. Stalactites and stalagmites in caves also provide temperature information thousands of years into the past. To go back even further in time, ice cores from Greenland, Antarctica, and glaciers are used. They contain small samples of the past atmosphere trapped in little bubbles in the ice. From these small bubbles direct estimates of carbon dioxide levels can be extracted, along with proxy air temperature information. Together, these ice core data provide clear evidence of the link between greenhouse gases and temperature over the past several hundred thousand years. Going back even further in time, sediment cores yield climate information over millions of years. What do all these paleoclimate records show? They show that carbon dioxide levels are higher now than they’ve been in at least several hundred thousand years and that current temperature levels are unprecedented for at least 500 years and likely for 1000 years or longer.

NOAA recently released its annual State of the Climate report and this report focused on collating a set of ten key measurements to give a multifaceted picture of the evidence of global warming. All ten very clearly indicate a strong warming trend over the past several decades. And that is the point that I want to leave you with. Climate science is a bit different than a lot of other sciences. We can’t do several laboratory tests to verify our theories because we have only one laboratory – earth. However, we can look at several different pieces of evidence and see if they convey a cohesive picture of the state of the climate. And indeed they do. Millions of pieces of dozens of different types of data, collected and analyzed by thousands of scientists over hundreds of years show one thing: the earth is unequivocally warming.


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Weather vs. Climate: A Race Between the Tortoise and the Hare

4/22/2010

By: Walt Meier, The National Snow and Ice Data Center

We’ve recently reached the annual maximum for Arctic sea ice extent. This year the event has caused a bit of a stir because of the lateness of the event. Normally, the maximum is reached sometime between late-February and mid-March, but in the past it has occurred as early as mid-February and as late as near the end of March. This year the maximum occurred on March 31, the latest date we’ve seen since our satellite data began in 1979.
Surely this means that Arctic sea ice is recovering and things are looking up, right? Actually, no. There is actually no correlation at all between the timing of the maximum and what happens during the summer. This is because any ice that forms late in the growth season is very thin and melts very quickly. In other words, the late new ice we saw this winter will be long gone by the time summer comes around.
What is the reason for the late-season ice growth? Simply put, it is the weather. At the ice edge in winter, the ice responds very quickly and directly to changes in weather pattern. If a cold snap with winds pushing the ice edge southward occurs, you get increasing sea ice extents. If unusually warm temperatures with winds pushing the ice edge northward occurs, the ice will retreat quickly. Eventually the warmth of the rising sun in the Arctic wins out and the ice retreats, but there is a period during February and March when the vagaries of weather control a lot about how far south the Arctic ice extends.
When assessing climate change, we’re talking about long-term trends. Carbon dioxide acts slowly over many years to increase temperatures. However, on top of this slow rise there are the ups and downs of weather and even short-term climate variations. That is why we can get seemingly contradictory situations like the late maximum in Arctic sea ice or “snow-pocalypse” storms in Washington, DC in a warming world. These are short term weather events and are not at all an indication that global warming isn’t happening. In fact, under warming conditions, there may be an increasing frequency of extreme events (warm or cold); so more big storms may be supporting evidence global warming, though no single storm can or should be attributed directly to global warming.
Even more persistent patterns over several months or even years, such as the increase in summer sea ice extent over the record low in 2007 or relatively cooler global temperatures the last few years don’t mean that anthropogenic climate change isn’t happening. Our picture of human influence on climate develops over several decades and on short time scales the influence of natural variations in climate will still play a significant role.
Rather than looking at things that are easily influenced by weather, such as the timing of maximum ice extent or freak snow storms, it is better to look at long-term indicators. For sea ice, one such indicator is the ice thickness. What we are seeing here is that ice is continuing to thin and that the Arctic has become dominated by younger, thinner ice. There has been a little bit rebound of some older ice over the past couple years, due to natural variations, but we’re nowhere near conditions seen in the 1980s and earlier.
Climate and weather are a bit like the tortoise and the hare. Like the hare, the weather will quickly go this way and that, but in the end it is the methodical tortoise-like pace of anthropogenic global warming that will ultimately win out and affect human society most significantly in the future.

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Global Implications to Arctic Sea Ice Loss

12/28/2009

By: Julienne Stroeve, National Snow and Ice Data Center

Once people find out what I do for a living, they tend to first ask ‘Is global warming really happening?’ And after I explain my views on the evidence for global warming, including the loss of the Arctic sea ice cover, many often then ask ‘Why should I care if the Arctic loses its sea ice cover?’ Besides the bleak outlook for the polar bears, which does invoke emotion for many of us, most people I speak with don’t understand the global implications of an Arctic without its snow or ice cover. Thus it’s not surprising that even members of my own family don’t regard the changes happening in the Arctic as anything to worry about.
 
So let’s start with the basics. Because of the Earth’s orientation relative to the sun, there is an inequality in the amount of solar radiation received by the poles versus the equator. This temperature gradient is what drives our atmospheric circulation, transporting heat from the equator towards the poles. Any changes in the Arctic sea ice cover, such as transitioning towards ice-free summers, modifies the basic temperature gradients and hence the manner in which the atmosphere transports heat. What this means, is that changes in the distribution of Arctic sea ice will affect our weather patterns, resulting in changes in temperature and precipitation patterns that affect transportation, agriculture, forestry, water supplies and even our recreational activities. Thus, it could be that those of us who enjoy skiing in Colorado every winter may have to start traveling elsewhere to ski as Colorado becomes drier in the future (an outlook from several climate models).
 
Today we are already seeing amplified warming in autumn over the Arctic Ocean as a consequence of more open water at the end of the summer melt season. This warming is a direct result of the ocean absorbing more heat from the sun during summer in the expanding open water areas. Before the ocean can once again freeze up in winter, it must first release the heat it gained during summer. This is seen as strong warming of air temperatures directly over the regions of sea ice loss. However, atmospheric circulation can spread this warming out over the adjacent land areas, helping to further warm the permafrost that is likely to lead to the release of stored carbon and thus further amplify the warming. A warmer Arctic will also help to accelerate the melt of the Greenland ice sheet and other small ice caps and glaciers in the Arctic. Thus, although the melting of the Arctic sea ice does not directly affect sea level, the warming induced by the loss of the ice cover can affect sea level by enhancing the melting in places like Greenland. In 2008, Greenland lost about 280 gigatonnes of ice. One gigatonne is the amount of water consumed annually by the 8 million inhabitants of Los Angeles, California. Though it sounds like a lot, it is only a small fraction of the volume of the Greenland ice sheet. Complete melting of the ice sheet would raise global sea level by 7 meters.
 
We are also already seeing a shift in the storm tracks across the North Atlantic, affecting precipitation patterns over Eurasia. Then of course there is the way in which the melting sea ice impacts the Arctic Ocean. As the ice melts, freshwater is added and the ocean becomes less salty (less dense). Changes in ocean density in the Arctic can alter global ocean circulation, with the ability to affect fisheries and other marine resources worldwide. 
 
While there is no universal consensus at this point as to what spatial patterns of change will emerge, many modeling studies do suggest that changes will be significant and affect not only those living in the Arctic, but us folks at lower latitudes too. We should care.

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Returning to Greenland

9/10/2009

By: Julienne Stroeve, National Snow and Ice Data Center
 
Blurry-eyed, we arrived promptly at 5am at the Air National Guard Base in Scotia, NY. An officer quickly looked over our passports, and then directed us to a waiting room where we sat for three hours until it was time to board the C-130. I was excited to be returning to Greenland. It had been nine years since I had set foot on the ice sheet and to me it is one of the most beautiful places on the planet. I was also excited to fly the 109th again, even though it’s not exactly first class. Flying the 109th is the only way for scientists and their gear to get directly to Greenland from the US and it’s a bit of an adventure. We sit next to the cargo on “seats” made of red webbing hooked to the inside walls of the plane. Ear plugs or noise canceling headphones are a must. If you’re lucky, you can score a spot on top of one of the pallets and lay down if the load master doesn’t mind. If you’re really lucky, you will be invited to come into the cockpit to enjoy the views.
 
Our C-130 was equipped with new fuel efficient propellers, JATO rockets and super-fat Black Diamond Gigawatt skis. However, despite the improved fuel efficiency, it was necessary to stop and refuel on our way to Kangerlussuaq, Greenland. We enjoyed some ice cream and coffee courtesy of the airport staff at the Canadian Forces Base at Goose Bay, Newfoundland. After a couple more hours of flying, spectacular views of mountains and glaciers began to emerge as we reached the edge of the Greenland ice sheet. So great to be back! Greenland is the world’s largest island and is covered almost entirely by ice. At the Summit of the ice sheet, the ice is more than 3 km thick. If the entire 2.85 million km³ of ice were to melt, global sea level would rise by nearly 8 m, which would flood most of the coastal cities in the world. Thus, scientists are extremely interested in how the ice sheet will respond to future warming.
 
I returned this summer to Greenland as a field assistant to another scientist who is monitoring how much melt water is being discharged from glaciers at the edge of the ice sheet. This was the third year in a row that discharge rates were collected.   The Principal Investigator on the project is hoping that a relationship will exist between temperatures measured on the ice sheet at automatic weather stations and the amount of melt water being discharged. If a direct relationship can be established, scientists will be able to better understand and predict how the mass balance of Greenland will change in the face of global warming. The last several years have seen record melting, with melt expanding to higher elevations, and recent mass balance estimates from the US space agency's Grace (Gravity Recovery and Climate Experiment) satellite, launched in 2002, suggest an average loss of 195 km3 of ice per year between 2003 and 2008. However, during the extreme melt year in 2007, more than 350 km3 of ice melted in only two months. This is approximately equal to the amount of water consumed annually by the eight million inhabitants of the city of Los Angeles, California.
 
We arrive in Greenland just in time for dinner. Greenland is not an easy place for a vegetarian, but luckily tonight they had fresh salad prepared at the cafeteria along with the assortment of meat dishes, including musk ox burgers. After a night spent at the KISS (Kangerlussuaq International Science Support) dorm rooms, we filled the pickup truck with our equipment and drove to the edge of the ice sheet.   We set up camp on the tundra and quickly went to work. The discharge of a river is the volume of water that flows through it at a given time, and requires us to obtain three measurements: the height of the river, how fast the river is moving and the shape of the river bottom. We made these measurements at different times of day and at different points along the river so that we can better characterize the temporal and spatial variability in the discharge rates. Pressure sensors are also kept year around in the river to measure water pressure, and in the air to measure air pressure. The difference between the two pressure measurements reveals the water depth between the surface of the water and the depth to the sensor in the river. We made sure to download the data collected by these sensors before we left so that we can analyze the data over the winter.
 
Camping next to the ice sheet was wonderful. My previous field work experience in Greenland involved camping directly on the ice sheet at about 1 km elevation. When you are working and living on the ice sheet you are surrounded by white, flat ice. Working “next” to the ice offers fantastic views not only of the ice sheet and its outlet glaciers, but also the mountains, and the lakes and rivers. Everyday we saw herds of musk ox and a lone caribou. An arctic fox was briefly spotted. One day we drove to the “end of the road” and walked on the ice sheet for about a half an hour in the rain. On our last day we took a hike to the base of Russell Glacier. An automatic camera is installed here as part of the extreme ice survey.  The glacier advances at a rate of about 25 meters every year and frequent calving events occur. We spent more than an hour marveling at the ice wall, getting as close as possible to the glacier while trying to keep a safe distance to avoid ice fall. 
 
This summer has been a bit cooler for most of the Arctic than the last couple of years, but still warmer than normal. The Arctic sea ice extent remains anomalously low, and will end up as the third lowest measured since satellite measurements began in 1979 (with 2007 being the lowest and 2008 being the second lowest), strengthening the downward trend in the summer sea ice cover. It is likely that melt in Greenland this summer will have continued the trend toward expanded melt extent and longer melt duration. There is no doubt that the Arctic climate is changing faster than the rest of the world, and changes occurring in Greenland are already having a considerable effect on sea level. But it’s not only sea level rise that is a concern. The Arctic acts as the Northern Hemisphere’s refrigerator, helping to regulate the Earth’s climate. As the snow and ice continue to melt, the planet will heat up even more, changing the atmospheric and oceanic circulation patterns of our planet that affect our weather. A recent report released by the World Wildlife Fund states that global feedbacks from Arctic climate change is already evident, and that ambitious constraints on greenhouse gas concentrations are needed now to avoid dangerous human interference with the climate system.
 
We don’t know yet what the data we collected will tell us. But we all felt grateful for the opportunity to spend some more time in this beautiful and remote place. The C-130 flight back home was especially memorable as I was invited to sit in the cockpit during the 100 km, low-altitude flight through the Kangerlussuaq Fjord. It was absolutely amazing!

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A Sense of Urgency

4/24/2009

By: Julienne Stroeve, National Snow and Ice Data Center

Indigenous people in the Arctic live under the flags of many countries, but their communities share a common theme: rising concentrations of atmospheric greenhouse gases are causing dramatic changes in their environment that are threatening their ability to live and prosper in the Arctic. The sea ice that they depend on for their livelihood and cultural identity is rapidly disappearing. In all regions of the Arctic and during all calendar months, the ice cover has been shrinking for decades. This month the National Snow and Ice Data Center and NASA announced that the sea ice is thinner now than it has been in the last 30 years, suggesting yet another large melt out of the ice during summer. Some scientists predict the Arctic Ocean could be temporarily ice-free during summer by 2030. Others are more extreme in their views, suggesting 2012 as a date when seasonally ice-free conditions will occur. This is something that hasn’t happened in probably at least 125,000 years.

Although the loss of the Arctic sea ice cover is currently our poster child of climate change, many other profound changes are happening in the Arctic that impact the people and animals that call the Arctic their home. During the same time the summer sea ice cover has declined by 40%, the Greenland ice sheet and most Arctic glaciers have been losing mass. Permafrost has thawed, vegetation zones have shifted, and air temperatures have risen faster than anywhere else in the world. The rapid pace of climate change in the Arctic has also resulted in increased transmission of invasive species, coastal erosion, greater industrialization and rapid social change. In many communities, rapid climate change is fueling already existing problems, such as contamination and other public health issues, food security, socio-economic inequalities and cultural preservation. And things will likely only get worse. The Arctic is believed to be a large storehouse of natural resources and many are rushing to explore and develop vast expanses of the Arctic. The combination of climate change and increasing natural resource development will transform this once remote and pristine area into a new region of importance to the global economy.

I do believe if we had the magnitude of environmental changes happening in our own backyards, as do the people in the Arctic, we would all be demanding our governments impose tight restrictions on greenhouse gas emissions. Yet many of us still feel detached from the changes currently underway in the Arctic. However, it is not only the people and animals living in the Arctic that will be impacted. We know that melting of land and ocean ice will have implications for the rest of the world in terms of rising sea levels, and altering temperature and precipitation patterns. It is time for scientists working in the Arctic to raise the level of awareness about the Arctic and its role in climate change, because processes unique to the Arctic, including feedbacks from melting snow and ice and terrestrial gas releases will have a dramatic effect on the global scale.  In addition, I believe action formulated to address Arctic issues must first begin from an understanding that indigenous people of the Arctic occupy a central role with valid interests and they must be involved in any decisions that will ultimately affect their well-being. Too often scientific, conservation and economic efforts are driven by interest outside the Arctic. The time is now for us to learn from their experiences and work together to preserve the Arctic and slowdown the rapid pace of environmental changes unfolding, because ultimately, it is not just the people in the Arctic that will be impacted.  

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Time for Reflection

2/27/2009

By: Mark Serreze, National Snow and Ice Data Center

As some readers may be aware, the global and annually averaged surface air temperature for 2008 ended up somewhere in the top ten over the instrumental record, yet somewhat cool when compared to the past decade. This global average temperature is of course our most basic measure of the climate state. I say that 2008 was somewhere in the top ten warmest because the exact ranking depends on the data set. Several climate centers around the world have compiled global average temperature time series, each with slightly different approaches to area averaging, treatment of urban warming biases, and other factors. Each center boasts that theirs is the best record. One sees the same sort of healthy rivalry between climate modeling centers. Every center likes to think that they have the best, most reliable model.

The global warming skeptics have predictably jumped all over the fact that 2008 failed to set a new record. Global warming has ended, they say. I wish that their claim held water. The major reason it was a little cool is mostly because of the natural variability that always has and always will be with us. Specifically, we were in the midst of a La Nina. In La Nina conditions, sea surface temperatures in the tropical Pacific tend to be cooler than on average, and the fairly cool temperatures are reflected in the global average surface temperature. This contrasts with El Nino conditions, when tropical Pacific waters are warm. The reason that 1998 still stands out as especially warm is because there was a strong El Nino. During 2008 we were also close to the minimum in the 11 year solar cycle. During the solar minimum with few sunspots, the total output of solar energy is a touch below normal. While it's a pretty small climate forcing, the 11-year solar cycle does indeed have an expression in temperatures.

At last check, the El Nino conditions that we’ve been in are starting to weaken. While the present solar minimum has been somewhat unusual in its duration, there is every expectation that we’ll come out of it soon. The next five years or so ought to be very interesting global temperature-wise. There has been some speculation that the continued rise in atmospheric greenhouse gas levels, coupled with disappearance of the temporary cooling factors just mentioned, may lead to a new record high global mean temperature in the next five years or so. Since I already have some egg on my face from last summer (contrary to my speculation, the area around the North Pole did not become free of sea ice), I’ll refrain from participating in betting pools for awhile.

Like tens of millions of others in our nation, have watched the still unfolding economic meltdown with a sense of shock and awe. Hopefully in the end, something good will come of it. The opportunity is now there to start embracing green technologies and conservation, key stepping stones for building a sustainable future for ourselves, our children and our planet. There is still time for us to turn our problems around and become proud stewards of the earth. We need to start today.

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Cautious Optimism

12/19/2008

By: Mark Serreze, The National Snow and Ice Data Center

With the nation in the throes of the economic meltdown, it's important that our elected leaders don’t lose sight of the looming problem of climate change. President-elect Obama will certainly have to focus much of his initial energy on addressing the economy. Still, this is the first time I can recall feeling even cautiously optimistic that the United States will finally show some leadership in addressing fossil fuel emissions.

There are, of course. many roadblocks ahead. While the $4.00 per gallon gasoline we were paying last summer was painful to the pocketbook, there was a silver lining in that it got people talking seriously about environmental technology and actually practicing conservation. Now that gas is cheap again, in part due to conservation, it may be all too easy to slip back into old habits and mind sets.

I'm growing a bit concerned over some of the increasingly strident activity on the part of so called "global warming skeptics." Perhaps the cheap gasoline of late has emboldened them. While this last summer saw sea ice extent in the Arctic fall to its second lowest level ever recorded, the fact that it didn't eclipse the old record set in 2007 has been hailed by some as a "great recovery." Conspiracy theories abound. This October, due to human error, erroneously high temperatures were posted for a number of stations in Siberia. This meant that the calculated global average surface temperature in October was higher than it really was. It seems that some temperatures for September were inadvertently placed in the data column for October. The error was quickly caught and the correct data posted, but not before the blogosphere came alive with accusations of temperature tampering by nefarious climate scientists. This is a common sort of ploy, for if the scientists seem to be fudging results, the whole idea of global warming must be cast into doubt. Readers may recall that we had a situation at our center this past summer, when we were accused of fudging numbers on sea ice extent.

The global average air temperature for the year 2008 looks like it will end up being a little cooler compared to recent years. This is just the latest expression of the natural climate variability that has and always will be part of the picture. Nevertheless, it’s a sure bet that the skeptics will have a field day when the final numbers come out, with claims that global warming has ended. I assure you, it has not.

The biggest danger in ignoring greenhouse gas emissions, of course, is that the longer we wait, the bigger the problem that has to be faced down the road. If we were to stabilize the atmospheric carbon dioxide concentration at its present-value, global surface temperature would stabilize in perhaps 30 years at a level with which we can deal. What concerns me is what is looming out there 50 or 100 years from now if no action is taken. My impression is that Mr. Obama understands the issues. Initial steps to curb greenhouse gas growth are likely to be small. However, taking even small steps can help to both build a path toward the future and find the courage to take bigger steps.

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Confused Thinking about Natural Cycles

10/17/2008

By: Mark Serreze, The National Snow and Ice Data Center

The basic measure of global climate change is an extended trend in the globally and annually averaged surface air temperature. A commonly-heard argument from those skeptical of a human role in global climate warming over the past 100 years is that it's just part of some "natural cycle."  Let's see how the argument falls apart.

We must first acknowledge a terminology problem.  A "cycle" suggests a regular or quasi-regular oscillation.  While the regular changing of the seasons fits this description, we of course don't view the transition from summer to winter as global climate change (remember also that summer in one hemisphere is winter in the other!).  A better example is the waxing and waning of continental ice sheets over the past two million years, occurring on a roughly 100,000 year cycle and associated with substantial changes in global mean temperature. However, we often toss events such as global cooling associated with the Little Ice Age (17th and 18th centuries) into the "natural cycle" category even though they are isolated phenomena. Hence, when the "natural cycle" argument comes up, remember that what's talked about isn't necessarily cyclic, just natural.

Where the skeptics typically start getting into trouble is failing to recognize that natural global climate change still needs to be forced by something. Global climate can't just change by Voldemort being in an especially bad mood. Take the waxing and waning of the continental ice sheets.  The climate forcing is variations, over tens of thousands of years, in the tilt of the earth's axis, in the timing of the equinoxes and in the earth-sun distance.  These combine in ways that influence the amount of solar energy striking the earth's surface at different times of the year and at different latitudes.  They are predictable from orbital mechanics.  Particular combinations favor initial Northern Hemisphere cooling and ice sheet growth, kicking in climate feedbacks that cause further cooling.  Contrast this with the Little Ice Age, which was probably caused by a slightly dimmer sun, perhaps working in combination with a string of volcanic eruptions - volcanic eruptions are known to cause temporary global cooling because of all the sulfur dioxide that they throw into the stratosphere.  I say "probably caused" not because of any fundamental shortcoming in our understanding about how the climate system works, but simply because we don’t have the data to really pin down what the forcings were back then.

This sets the stage for pointing out the logical flaw in the skeptic's argument:  1) If you're  going to argue that global warming over the past 100 years is just part of a natural cycle, you still have to come up with an associated  climate forcing; 2)  There is no evidence of any natural forcing (e.g., a brighter sun, changes in orbital geometry)  that can explain the warming;  3)  Even if you argue that there is some mysterious natural forcing at work that nobody has yet identified  (Voldemort?), you are then forced to argue that the rise in atmospheric greenhouse gas concentration, which has a substantial and readily measured forcing, has no effect, even though it explains the observed warming quite well.  Simply put, you can't have it both ways.                   

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Summer 2008: Hot Times in the Arctic

9/5/2008

By: Mark Serreze, The National Snow and Ice Data Center

What a summer it has been.  The Arctic is heating up and it's not just the sea ice that's melting fast.  Still, melting sea ice is where we need to start, as everything else turns out to be related in some way.

As reported by the National Snow and Ice Data Center (NSIDC) on Tuesday, August 26, Arctic sea ice extent had fallen to its second lowest level since regular monitoring began by satellite.  Since there are still a couple of weeks left to go in the melt season, we may yet beat the standing all time record set in September of 2007.   My guess is that we'll fall shy in a photo finish.   Assuming that I'm right, we're sure to hear skeptics remarking on the great recovery from worst to only second worst.   

Some people don't want to accept the overwhelming evidence of global warming and as such will go to great lengths to try and discredit the messengers, the messengers being the scientists.  As a case in point, a couple of weeks ago, NSIDC was accused in the blogosphere of fabricating results - fudging our numbers on sea ice extent to make things look worse than they really are.  We of course don't do this sort of thing, preferring to let the data speak for itself.  After a well-orchestrated counterattack led by one of our top scientists, Walt Meir, who unlike the accuser, has a solid grasp of the facts, the fabrication story was retracted. But not before a number of skeptic blogs picked up the fabrication story, only to end up embarrassing themselves. As they say, live by the blog and die by the blog.

As the sea ice cover continues to shrink, the Arctic becomes more accessible, not just to tourism and commercial shipping, such as through the Northwest Passage, but to exploitation of oil wealth at the bottom of the ocean.   Indeed, a number of new oil drilling permits have been issued in the Beaufort and Chukchi seas, north and northwest of Alaska.  In recognition of growing accessibility and oil operations, the United States Coast Guard set up temporary bases this summer at Barrow and Prudhoe Bay, AK, from which they conducted operations to test their readiness and capabilities,  such as for search and rescue.  The Canadians have been busy showing a strong Arctic presence.  Their view is that they own the Northwest Passage through the channels of the Canadian Arctic Archipelago.   The United States does not agree.  In August, a German crew traversed the Northwest Passage from east to west in one of their icebreakers, the Polarstern.  While they weren't trying to challenge the Canadians (the traverse was a planned activity of the International Polar Year, an ongoing international effort to better understand the polar regions, especially the Arctic), it was yet another demonstration of how the once largely inaccessible Arctic is opening for business. 

For some, the changing Arctic spells good economic times. As always seems to be the case,  however, the biggest victims of change will be those least responsible for it,  including peoples of the north who are seeing their traditional, time-honored ways of life disrupted,  and that very symbol of the Arctic, the polar bear. 

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The North Pole is Safe This Year

8/7/2008

By: Mark Serreze, The National Snow and Ice Data Center

Back in late June, there was noise in the blogosphere and standard media over concerns that the North Pole might become sea-ice free by the end of this summer.  While this possibility had been tossed around in the science community as early as April, it gained wider attention after some comments in "National Geographic Online" by a colleague up in Canada, a hardy fellow who had spent a chunk of spring observing the state of the sea ice from the deck of an icebreaker. As the story broke, I got a bunch of phone calls asking for my views on the matter.

Why were we thinking about an ice-free North Pole?  It was clear by last April that much of the Arctic Ocean, including the region around the North Pole, was covered with rather thin ice. Since it takes less solar energy in summer to completely melt thin ice than thick ice, it followed that there was a good chance for ice-free conditions developing at the Pole.  While we know that the Arctic ice cover has thinned over the past 25 years or so, we'd never seen so much thin, vulnerable ice.   To clarify, nobody was arguing that the entire Arctic Ocean would become ice free - just that for the first time in recorded history, the Pole itself might have no ice, such that you could get there in a canoe. From the science perspective, the North Pole is just another point on the globe.  On the other hand, the North Pole has symbolic meaning.  There is supposed to be ice there.  The possibility that there might be none struck a nerve.  

I stated back in late June that the chances of the North Pole melting out were perhaps 50/50. The big unknown was how the weather patterns would set up this summer.  Like the number that comes up when throwing dice, this was something pretty much up to chance.   Last summer saw a weather pattern ideal for melting sea ice.  We had a situation in which much of the Siberian side of the Arctic Ocean was influenced by persistent warm winds from the south.  At least so far, the weather patterns this summer have been different, leading to somewhat cooler conditions.  The pace of summer ice melt has been correspondingly slower.  It seems that the North Pole will be safe for this year.  This is one case where I'm glad to admit that I was wrong!

When will the Arctic Ocean (including the North Pole of course) become ice free in summer? It's one of these things that we just can't pin an exact year on.  The problem is that while Arctic sea ice extent is on a clear downward trend, natural climate variability will always be there to introduce blips in the record that complicate the picture.  Last year, the "perfect storm" worked in conjunction with a thinning ice pack to give us the lowest summer ice extent ever seen.  This summer, a somewhat cooler pattern has prevailed, so we've seen some recovery.  While Santa Claus can hence relax for another year, he nevertheless ought to be shopping around for a new home.   

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What Climate Scientists Do

7/11/2008

By: Mark Serreze, National Snow and Ice Data Center

I live in Boulder, Colorado, and work at the National Snow and Ice Data Center, part of the University of Colorado. When it comes to climate research, Boulder is a beehive of activity.  The city is home to hundreds of climate scientists who variously work at the University, the National Center for Atmospheric Research (NCAR), offices of the National Oceanic and Atmospheric Administration (NOAA) and various private firms.

Like everyone else, we struggle to pay the rent or mortgage, try and juggle family and work, go to the grocery store,  worry about eating too much fat, and root for the local sports teams.  While we have many different personalities, we share the passion of trying to figure out how the global climate machine works and where it is headed.   Climate research is still a male-oriented occupation, but the landscape is quickly changing.  It’s a field in which personal plumbing is entirely irrelevant.  You are respected for your insights and contributions. 

Some of us are observationalists, that is, folks who are interested in what climate measurements are telling us.  Observationalists range from field oriented types to those who are happy crunching numbers in the office.  Hard-core field types like to travel to the far points of the globe to collect weather and climate data or perhaps set up new instruments.  There's a big project going on right now called the International Polar Year,  which coordinates efforts of scientist from many different countries to better understand climate change in the polar regions,  especially the Arctic.  As part of this effort, I was briefly in Barrow, Alaska, helping to conduct measurements of Arctic snow cover and sea ice thickness.  My back still aches from the experience.      

When you hear of things like how the global average temperature for 2007 stood in comparison with other years in the historical record (second highest according to the analysis at NASA), or how much of the Arctic Ocean was covered by sea ice in September 2007 (the least ever measured, by far), you know that the number-crunching observationalists have been at work.  They take great pride in reducing millions of numbers down into a handful of meaningful ones.  Analysis of satellite data is a big part of climate research that falls under the general purview of the observationalist.  Satellite data can gives us information on a wide range things, including, but not limited to, atmospheric, land surface and sea surface temperature,  vegetation cover, global snow cover and sea ice extent.  We'd be lost without it.

Other climate scientists are modelers.   There are a lot of these in Boulder, especially up at NCAR.  They tend to be happiest sitting behind their computer screens, developing ever more complex and realistic numerical models of the global climate system or parts of it (like the ocean).  If you hear them cursing, it's probably because their code won't compile. 

Still more are generalists - those who combine observational and modeling perspectives and may even do a bit of field work.  I place myself in this category.   While I'm a "jack of all trades", I'd be the first to admit that I'm really a master of none.        

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There is Indeed Scientific Consensus

6/12/2008

By: Mark Serreze, The National Snow and Ice Data Center

 

Are human activities responsible for observed global warming over the past century or is this just part of some natural climate cycle?  From some media reports and websites, it's easy to get the impression that the science community can't come to any consensus. Be assured that there is indeed consensus.

 

Building from my first blog entry (4/18/08) an extended change in global temperature requires a climate forcing.  For example, if the sun were to shine a little brighter, the global mean temperature would rise because the earth's surface would receive more energy.  Big volcanic eruptions force a temporary drop in global temperature, because aerosols thrown up into the stratosphere prevent some of sun's energy from reaching the surface.  Increasing the atmospheric concentration of carbon dioxide must cause warming, because this gas absorbs some of the longwave radiation emitted by the surface and directs it back downward.  It doesn't matter that you can't see, smell or taste carbon dioxide - increase its concentration, and global temperature must rise.    Global warming has been especially strong over the past 30 years.  This is clearly linked to a strong and growing forcing from carbon dioxide and other greenhouse gases like methane. 

 

Debate in the informed science community is in the details.  Just how much will global temperature rise in the next 50 or 100 years?  While part of the difficulty in answering this question lies with uncertainty in the strength of climate feedbacks, the bigger issue is uncertainty in future rates of greenhouse gas emissions.  This is tied to uncertainty in human behavior.  Another area of debate is how changes will be expressed regionally.  Some regional responses, such as outsized warming of the Arctic (see my post for 5/16/08) are a sure bet.  On other important issues, such as how much precipitation will change in the American Midwest, we are less certain.

 

Why can one get the impression that the science community has no common voice?   Partly it's our fault, for scientists, by their nature, tend to couch things in uncertainty.  While we know that global climate will continue to warm, and we know why, we have a tendency to dwell on the details like those just discussed.  It often leads to the impression that we don't agree on even the major points.  As a group, we are aware of this problem and are trying to become better communicators. 

 

Another issue is that while reporters try (and rightly so) to present a balanced argument, global climate change is an issue where attempts at balance often end up promoting bias. What basically happens is that while you could have a hundred scientists all saying the same basic thing (climate change is happening and we know why), the one contrarian that gets interviewed gets equal billing. 

 

Finally, there are groups that perceive global warming as a threat to the status-quo and therefore do whatever they can to seed doubt and confusion.  This can range to clever cherry picking of facts to publishing blatant misinformation.  If you don't believe this, just think back to the activities of the tobacco lobbyists.         

 

 

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The Arctic: Fast Track of Change

5/16/2008

By: Mark Serreze, The National Snow and Ice Data Center

 

Much of our understanding of how climate change will unfold through the 21st century is based on complex computer models that link together physical processes in the atmosphere, ocean and land. Global climate models are run by groups all over the world.  The model I'm most familiar with is operated by the National Center for Atmospheric Research here in Boulder, Colorado.

 

Every one of these computer models tells us that as we increase the concentration of carbon dioxide in the atmosphere, the Arctic region will respond first and the warming there will be more pronounced than elsewhere on the planet.  In other words, the Arctic should be the early warming system, or the "canary in the coalmine". While there are a number of reasons why the Arctic should be so sensitive, the most important is the albedo feedback mentioned in my last post. If we increase the concentration of greenhouse gases and temperatures rise a bit, some of the Arctic's highly reflective (high albedo) snow cover and sea ice melts, exposing darker underlying surfaces which absorb more of the sun's rays.  This in turn fosters even more warming and more melt of snow and sea ice.  Sea ice is the floating ice that covers the Arctic Ocean, with its extent waxing and waning with the seasons.

 

Evidence that the Arctic was already starting to change grew through the 1990s. Were these changes driven by greenhouse warming or were we just seeing natural variability in the climate system?  We weren't sure - part of the "attribution problem" was that natural variability is quite pronounced in the Arctic. It was probably around the year 2000 when things started to fall into place.  By then, it had become clear that the changes unfolding in the Arctic were much too persistent and coherent among different parts of the system to be dismissed as natural climate fluctuations.  

 

Eight years have passed, and the emerging issue that I am faced with as a climate scientist is coming to grips with the surprising rapidity of change. In many ways, it seems that reality has exceeded expectations, and that our vision of the Arctic's future is already upon us. The most visually striking evidence of rapid change is the Arctic's shrinking sea ice cover. While climate models tell us that sea ice extent should already be declining in response to greenhouse gas loading, the observed loss is much faster - we are perhaps 30 years "ahead of schedule". Last summer saw the least sea ice every recorded by satellites.  While we knew that Arctic warming would be outsized compared to the rest of the planet, this so-called Arctic amplification is also ahead of schedule and is growing.   Permafrost, perennially frozen ground that underlies must of the Arctic lands, is warming then thawing. Areas of formerly treeless, windswept tundra are being taken over by shrubs.  The Greenland ice sheet is stirring in ways quite unexpected ten years ago, with disturbing implications for sea level rise.

 

In summary, the Arctic is on the fast track of change.

 

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The Basics of Climate Change

4/18/2008

By: Mark Serreze, The National Snow and Ice Data Center

he earth has a natural greenhouse effect, without which it would be too cold to support life.  Greenhouse gases in the atmosphere trap some of the infrared (or long wave) radiation emitted from the surface, preventing it from escaping to space, making the Earth's surface much warmer than it otherwise would be.   Greenhouse gases can hence be thought of as acting like a “blanket” for infrared radiation.  The most important of these greenhouse gases is water vapor, accounting for about half of the natural greenhouse effect. Carbon dioxide and other gases such as methane contribute another quarter.  Clouds also absorb and emit infrared radiation, and account for the remainder.

For thousands of years, the planet has been close to a state of radiative equilibrium, in which it returns as much energy back to space (as infrared radiation) as it absorbs from the sun (as solar, or shortwave radiation).  Radiative equilibrium implies a steady global surface temperature. However, starting around the industrial revolution, largely as a result of fossil fuel burning, atmospheric concentrations of greenhouse gases started to rise. This has caused a positive radiation imbalance (more energy coming in than going out), to which the response must be a rise in the global surface temperature. The extent to which an agent can throw the planet out of radiative equilibrium to initiate warming (or cooling) is termed radiative forcing.  In the year 2005, considered with respect to pre-industrial conditions, there is a radiative forcing of about 1.6 Watts per square meter.  This is the largely the net result of the warming effect of increased greenhouse gases.

The best estimate is that for every Watt per square meter of positive radiative forcing, the global surface temperature rises by about 0.75oC. It is a fairly big number.   This is because of positive climate feedbacks that amplify the temperature response.  The most important of these feedbacks is associated with water vapor.  Initial warming fosters more evaporation, and the warmer atmosphere will carry more water vapor.  However, as water vapor is a greenhouse gas, this causes further warming. Another is the albedo feedback.  Albedo refers to the reflectivity, or "whiteness" of the surface.  As the temperature rises, some of the earth's high albedo snow cover and sea ice melts, exposing darker underlying surfaces.   As albedo drops, more of the sun’s energy is absorbed at the surface, causing further warming. 

In summary, we can think of global climate change in terms of a three step process of forcing, feedback and response.  Today's radiative forcing would yield an eventual increase in global surface temperature of about 1.2oC in the next 30-50 years.  Since 1900, the global temperature has already risen by about 0.7oC, implying another half a degree still ahead of us.   This lagged response to the forcing reflects heat still "in the pipeline."  This is heat stored in the planet's vast oceans that will eventually come out and further raise the surface temperature.  If we didn't change the radiative forcing, most of the additional warming would take place in the next 30 to 50 years. 

However, even further warming is in store, for as greenhouse gas concentrations continue to grow, so will the radiative forcing.  This growth, in concert with the delayed response of the climate system to forcing, also means that equilibrium is a constantly moving target.  Because of regional expressions of climate feedbacks, changes in atmospheric circulation and a suite of other factors, the magnitude and rate of warming and changes in other key elements through the 21st century  (such as precipitation) will not be uniform across the planet.  For example, the Arctic will warm the most.  Because there is still plenty of snow and ice in the Arctic that can be melted, the albedo feedback will be very strong there.

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