Thursday, October 14th, 2010
At the 2009 meeting of the American Geophysical Union, renowned climate scientist Richard Alley (Penn State) gave a keynote address, The Biggest Control Knob: Carbon dioxide in Earth’s Climate History, in which he used a variety of paleoclimatological proxy data to show how CO2 changes over much of Earth history have exerted a strong influence on global temperatures.
In this week’s issue of Science, Andrew Lacis and colleagues published an article, Atmospheric CO2: Principal control knob governing Earth’s temperature (abstract only; subscription required), following up on this theme. Unlike Alley’s talk, which mainly focused on the role of CO2, this team starts by going after water vapor and confronting a widely held perception that it is the dominant greenhouse gas:
It often is stated that water vapor is the chief greenhouse gas (GHG) in the atmosphere. For example, it has been asserted that “about 98% of the natural greenhouse effect is due to water vapour and stratiform clouds with CO2 contributing less than 2%”. If true, this would imply that changes in atmospheric CO2 are not important influences on the natural greenhouse capacity of Earth, and that the continuing increase in CO2 due to human activity is therefore not relevant to climate change. This misunderstanding is resolved through simple examination of the terrestrial greenhouse.
Water vapor is a main reason why the world has a pleasant and life-sustaining average temperature of 16 degrees C. Based on the distance of Earth from the Sun, physics tells us that Earth should be about 0 degrees C—a giant snowball hurling through space. The reason why we are warmer than this is because of the natural envelope of greenhouse gases, including water and CO2 that absorb longwave heat radiating from the surface. This warms the surface of the planet just like a thick blanket keeps your body heat near your skin on a cold night.
In round numbers, water vapor accounts for about 50% of Earth’s greenhouse effect, with clouds contributing 25%, CO2 20%, and the minor GHGs and aerosols accounting for the remaining 5%.
So water vapor and clouds make up about 75% of the greenhouse effect, which sounds like the definition of “the dominant greenhouse gas” to most of us. How does one show that CO2 really is more important than water vapor as a primary greenhouse gas driving temperature change when it looks like water is so important?
Saturday, March 13th, 2010
Most people have heard about the potential positive feedback of soil carbon on climate: As temperatures warm, soil microbes are more active and permafrost begins to thaw–both of which can hasten decomposition and the release of CO2 to the atmosphere. This, in turn, has the potential to accelerate warming.
A lot of us who study climate warming impacts in boreal and Arctic ecosystems are interested in this problem. There are a few things we keep an eye on:
All of these questions are active areas of research. Increase any of them, and you have the possibility of strengthening the positive feedback. The third one is particularly interesting. The more we study and inventory soil carbon at high latitudes, the more we revise upwards the estimate of soil carbon.
Here’s an example: The atmosphere contains about 750 gigatons of carbon. When I was in grad school back in the early 90′s, we thought that boreal and arctic soils might have stored around 350 gigatons—about half the atmospheric content. With the discovery of extremely carbon-rich yedoma soils in Siberia, we learned that this number might be a serious underestimate. And as we learn more about soil carbon stored in deeper, harder-to-sample permafrost soils, we are coming to the realization that high-latitude soils may store between 1000-1700 gigatons—substantially more than the atmosphere (here’s one example).
Let’s say for illustration that the real number is 1500 gigatons. This means that warming would only need to cause a loss of 1/2 of 1% of this soil carbon to release 7.5 gigatons—roughly the total amount of fossil fuel carbon released worldwide each year. Thus, small changes in decomposition of a huge soil carbon pool can lead to carbon releases that rival anthropogenic emissions.
In a forthcoming issue of Global Biogeochemical Cycles,1 Jennifer Howarth Burnham and Ronald Sletten further illustrate that the more we sample, the more this soil carbon number goes up.
Focusing on Greenland, they dug 55 soil pits and measured soil carbon. Then, they extrapolated these estimates to the rest of the circum high Arctic by (1) linking soil carbon to certain vegetation types and (2) using satellite imagery estimates of the area of each vegetation type to estimate soil carbon for a much larger region.
Although the new number they produced is not large (12 gigatons), it is five times the previous estimate for High Arctic soils. It’s important to note that much of the High Arctic is a polar desert with little plant growth that could contribute to soil carbon, so it’s not surprising that more of the soil carbon is farther south—in boreal and subarctic regions.
One important caveat is that they only sampled surface soils that thaw during summer and are easy to sample. By omitting deeper permafrost soils, they probably underestimated the total.
And so we keep sampling…
1Burnham, J. H., and R. S. Sletten (2010). Spatial Distribution of Soil Organic Carbon in Northwest Greenland and Underestimates of High Arctic Carbon Stores Global Biogeochemical Cycles : 10.1029/2009GB003660
Photo credit: One of my photos from the Canadian Arctic that you can view on my flickr site
Wednesday, March 10th, 2010
The IPCC 2007 report projected a conservative sea level rise of about 18-59 cm by the year 2100.
Why conservative? Because it mainly accounted for things we know are happening and can measure well—like thermal expansion of the ocean and melting of land glaciers (see here for a discussion of the Kilimanjaro example). What it doesn’t do so well is account for all of the potential ways that the big ice sheets (Greenland and Antarctica) can contribute to sea level rise. Things like ice flow and mass loss are generally assumed to be constant, even though recent research papers discussed in previous posts (here and here) suggest they are accelerating.
Since the publication of the IPCC report in 2007, there have been several studies suggesting that sea level rise will be 1-2 meters or more by 2100 (one example here). One study looked at geological evidence for sea level rise during the previous interglacial period 125,000 years ago, which was 1-2 degrees C warmer than today. Their work indicated that there was a 95% chance that sea level rose by 6 meters (22 feet).
In a forthcoming issue of Geophysical Research Letters, Svetlana Jevrejeva and colleagues used statistical models to project sea level rise by 2100.1 But they also did something else interesting. They looked back several thousands of years to the most extreme events that could cause climate cooling—things like severe volcanic eruptions, which create stratospheric dust clouds that block sunlight.
If events like this were to happen again, they asked, would they cause enough cooling to be able to slow sea level rise caused by greenhouse gases?
The answer is no. There appears to be no natural factors like vulcanism that will significantly slow greenhouse-gas-driven sea level rise that we are already committed to or future sea level rise that we may experience if we continue to emit fossil fuels.
Excerpts (emphasis mine):
1Jevrejeva, S., J. C. Moore, and A. Grinsted (2010). How will sea level respond to changes in natural and anthropogenic forcings by 2100? Geophysical Research Letters : 10.1029/2010GL042947
UPDATE: RealClimate provides more explanation of the IPCC being too cautious about sea level rise.
Friday, February 12th, 2010
It’s been an incredibly busy week, which explains the dearth of posts. But good things are happening, which I look forward to sharing.
As most of you know, there’s an energetic, ongoing debate about environmental messaging. With polls showing waning interest in climate warming as a serious issue, there’s a sense that the battle is being lost.
I mentioned in an earlier post that it’s often assumed that climate change science speaks for itself. All we have to do is publish good science and show the public a bunch of data, and this will lead to a collective consciousness demanding action on climate warming.
It hasn’t worked out that way.
One main problem is the failure to connect with people on a personal level. Thinking about the environment is not just about climate or wild nature; it’s about human nature, human experience, the intersection of nature and culture, how we interact with one another—things squarely in the domain of the social sciences and humanities. In order for society to connect with contemporary environmental issues, it’s critical that these voices become part of this conversation.
Paul’s work is a beautiful illustration of how one artist has been able to put a human touch on climate warming. His show was packed with a hyped-up audience that cut across a wide swath of young and old.
Try doing that with a science seminar.
Amanda Little reminds us that there are no silver bullets for solving climate warming, only silver buckshot. Paul’s work (and the work of other popular artists like him) is a great example of one of those buckshot.
Photo Credit: Tiffany Gerdes, Bowdoin Orient
Thursday, January 7th, 2010
Most people have probably heard about positive feedbacks at high latitudes and why they matter:
A new study1 in the Early Edition of the Proceedings of the National Academy of Sciences (open access), indicates that other effects of forest changes might also matter.
Specifically, boreal forests and tundra may become more dominated by deciduous trees (ones that drop their leaves in autumn), which are usually found in warmer regions. What happens if we have a future Arctic dominated by these species?
Using a set of ecosystem and climate models, Abigail Swann and colleagues determined that a rise in deciduous forests would cause an increase in water vapor to the atmosphere (deciduous trees transpire—lose water through their leaves—more than conifers). This makes the atmosphere in the Arctic more laden with water vapor, which is a good greenhouse gas. This warming, in turn, induces further sea ice and snow loss, causing warming to happen more quickly. But wait, there’s more: Warmer, ice-free oceans also release more water vapor to the atmosphere, causing greenhouse warming to increase even more.
How big an effect? About 1 degree C in the Arctic, equivalent to increasing the atmospheric CO2 about 100 ppm in the atmosphere. They found that these changes in water vapor have about the same impact as the changes in reflectivity caused by the color of forest foliage overtopping snow in the tundra.
Things like this are reasons why when warming starts, it can accelerate faster than we think.
1Swann, A. (in press) Changes in Arctic vegetation amplify high-latitude warming through the greenhouse effect. Proceedings of the National Academy of Sciences
Saturday, November 14th, 2009
Greenland and Antarctica are two places that climate scientists are studying intensely because of the potential for significant sea level rise were they to melt. Over the past 20 years, scientists have used a variety of methods to track ice loss, and they have found that Greenland has been losing ice more rapidly over the past decade than it had in the 1990s. In fact, since 2004, ice loss has accelerated to such a high level that Greenland is now losing about 270 billion tons of ice per year. Greenland’s contribution to sea level rise has been about 0.13-0.74 mm/yr, or about 4-23% of global sea level rise observed from 1993-2005.
In this week’s issue1,2 of Science (subscription required), Michiel van den Broeke and colleagues used a couple of methods to confirm that this acceleration of ice loss is real and to understand why it’s happening.
The extent of ice in a glacier is like a bank account, but instead of money, we’re keeping track of ice. When inflows (precipitation = snow) exceed outflows (mostly due to melting and runoff), the ice sheet gets bigger, just like a bank account grows when deposits exceed withdrawals. We say that there is a positive surface mass balance. When outflows exceed inflows, then the ice sheet shrinks, and we say there is a negative surface mass balance.
They found that before 1996, Greenland’s ice sheet had a positive mass balance (getting bigger) because precipitation exceeded runoff. Between 1996-2004, precipitation and runoff both increased, and since these roughly cancel out one another, the ice sheet didn’t change much. However, after 2004, precipitation stopped increasing while runoff continued to rise exponentially. Mass balance has been negative for about five years now, with a cumulative mass loss of almost one trillion tons of ice in that span. Amazing.
The next big question, therefore, is what’s causing precipitation to change? Will it go back up, thereby reversing the ice loss, or will it remain the same or decrease, causing loss to continue accelerating? Nobody knows at this point.
1van den Broeke (2009) Partitioning recent Greenland mass loss. Science 326:984
2Bowdoin people can access the article here.
Thursday, October 22nd, 2009
A lot of us have been following the paleoclimatology literature examining changes in global temperatures and atmospheric CO2 over the past 60 million years, which can be deduced using different chemical signatures in ocean sediment cores.
One time period, in particular, is especially relevant to the discussion of rising CO2–a change between 33.5-34 million years ago (MYA) called the Eocene-Oligocene (E-O) transition.
What happened back then? Around this time was the first appearance of consistent polar ice on Antarctica. Before then, atmospheric CO2 levels were high enough that Earth’s climate was a hothouse, perhaps as much as 8-10 degrees C warmer than today. Antarctica was lush and green with forests.
The worry is that if we start approaching atmospheric levels of CO2 similar to those before the E-O transition, we may warm the climate to a condition where polar ice us unstable. That would be bad news because the loss of the Antarctic ice sheet would raise sea level by more than 60 meters.
This week, Paul Pearson and colleagues (who have done a lot of this great work) published a new article1 examining the E-O transition in more detail to see if it has any clues for our modern environmental challenges.
What did they find?
Thursday, October 15th, 2009
Here’s the bad news: