Arguments
Understanding climate feedbacks
Posted on 12 October 2015 by Guest Author
This is a re-post from Carbon Brief by Prof Eric Wolff, Royal Society research professor in the Climate Change and Earth-Ocean-Atmosphere Systems Group at the University of Cambridge.
This week, the Royal Society published a special journal issue on the topic of climate feedbacks. This edition collects together the papers based on presentations that scientists made at a Royal Society conference in December 2014, covering the latest science and uncertainties around climate feedbacks.
But before you delve into papers themselves, allow me to introduce the concept of feedbacks and why they're so important.
Significantly positive
Human activities, most notably those causing increases in the concentration of carbon dioxide, are changing the climate. On the current trajectory of carbon emissions, very substantial changes in climate are predicted by the end of the century.
If the rise in atmospheric carbon dioxide has no knock-on effects, calculating the size of those changes would be straightforward. If we doubled the amount of carbon dioxide in the atmosphere compared to pre-industrial levels, the Earth's temperature would eventually balance out about 1.2C warmer.
But reality is somewhat more complicated. Once the Earth begins to warm, it triggers other processes on the surface and in the atmosphere. These are climate feedbacks. "Positive" feedbacks will strengthen the warming, while "negative" ones will weaken it. The most recent report from the Intergovernmental Panel on Climate Change (IPCC) concluded that the combined effects of all feedbacks is likely to be significantly positive.
Uncertainty around the magnitude of these feedbacks is the main reason scientists use a range rather than an exact figure when estimating how much the Earth will warm up in the future, even when we prescribe the carbon emissions. And it's not just global temperature they affect - they also play a key role in regional climate, too.
Sunset at Vatnfjorden, Norway. © Sonja Jordan/imageBROKER/Corbis.
Fast feedbacks
Some climate feedbacks kick in quickly as temperatures go up. These "fast" feedbacks involve changes in water vapour, clouds and sea ice extent.
As temperatures rise, more water evaporates from the Earth's surface and is present in the atmosphere. As water is itself a greenhouse gas, higher concentrations in the atmosphere have a warming effect, creating a positive feedback. This process is well understood by scientists.
One example of a negative feedback is caused by changes in the rate that temperature falls the higher you go in the atmosphere. When the climate warms, it is expected that different altitudes warm by different amounts. The dominant change is in the tropics, where the upper troposphere should warm faster than the surface. This causes more heat to be lost than would be expected under uniform warming, so this "lapse-rate" feedback is negative.
More water vapour in the air will also affect the number and type of clouds that form. Here it gets more complicated: depending on their type and altitude, clouds can have a warming or cooling effect on the Earth.
How these effects balance out remains the major source of uncertainty in scientists' projections for near-term climate change.
There have been great advances in understanding how different cloud types respond to change. But compared to the units of space in a model of global climate, individual clouds are very small, so it is difficult to represent all the cloud processes that scientists know about. In our Royal Society special issue, Christopher Bretherton's paper looks into the latest science on this topic.
Another fast feedback is changing sea ice and snow cover on land. Bright white ice and snow reflect a large portion of the Sun's energy that hits the Earth's surface - they have a highalbedo. If sea ice and snow melt away, they leave behind a darker surface of ocean or soil, which reflect less energy, causing the surface to warm further.
This positive feedback is a crucial component in producing the biggest regional contrast in the response to climate change: Arctic amplification. Declining sea ice is causing the Arctic to warm twice as quickly as the global average. Scientists have tentatively linked the rapidly-warming Arctic to extreme weather in mid-latitude countries, such as the UK, US and Russia.
Warmer temperatures could also cause permafrost soils across the Arctic to thaw, with the potential to release their vast stores of organic carbon. Charles Koven and colleagues tackle this issue in their paper.
Thawing permafrost on the Tundra of Wrangel Island. © Jenny E. Ross/Corbis.
Longer timescales
Of course, some feedbacks take a little longer to make their presence felt. For example, the growth and contraction of vast ice sheets has been a major climate feedback from one ice age to the next throughout Earth's long history. The albedo effect applies in the same way as for sea ice, but changes in these vast blankets of ice don't happen as quickly.
There are good reasons to suppose there is a temperature threshold beyond which the loss of much of the Greenland ice sheet becomes inevitable. The future of the West Antarctic ice sheet is also uncertain, and because much of it sits below sea level, the ice sheet is vulnerable to warmer oceans as well as higher air temperatures. While changes in sea level will play out over millennia, actions taken or avoided in this century might commit the planet to seeing them happen.
Feedbacks triggered by rising carbon dioxide can also occur within the carbon cycle itself - a topic covered by Pierre Friedlingstein in his paper. More than half of manmade carbon emissions are currently absorbed by the ocean and the land biosphere. However, as the oceans warm, their capacity to store carbon could diminish. Likewise, how much carbon dioxide that trees, plants and soils can absorb is likely to be different in a warmer world. It is clear that positive feedbacks in the carbon cycle have been very important for the change from ice ages to warmer periods.
Finally, in modern times there is a new kind of feedback: humans. We are, in effect, a whole category of our own. How we change - or don't change - our behavior in response to climate change will play a major role determining in the scale of global temperature rise in the coming decades and centuries.
Climate sensitivity
Scientists use three parallel lines of study to estimate the strength of feedbacks.
First, there are studies that aim to uncover how different feedbacks actually work. These contribute the detailed knowledge scientists need to improve climate models. Second, there are modelling studies that show how the different feedbacks combine to affect global climate. And, third, there is research into Earth's past that provides examples of how the climate has responded to natural forcings, such as volcanic eruptions, or natural emissions of greenhouse gases.
Scientists have greatly improved understanding of many of the major feedback processes that have affected past, and will affect future, climate change. Quantifying the magnitude of the major feedbacks is still difficult, however. So, for the time being, scientists will still present climate projections as a wide range, but we're working on making it narrower.
This guest post is based on the following open-access article:
Wolff, E.W., Shepherd, J.G., Shuckburgh, E. and Watson, A.J. (2015) Feedbacks on climate in the Earth system: introduction, Phil. Trans. R. Soc. A, doi:10.1098/rsta.2015.0146.
Thanks for presenting these important studies. It sure would be nice, though, to have a better idea of exactly how they affect the range, and the most likely value, for climate sensitivity. It seems odd that the 'wide range' link in the last line pulls up an old slr chart that does not seem to incorporate the feedbacks discussed here.
A human positive feedback:
http://www.bbc.co.uk/news/science-environment-34501867
Yep, ignore the cause of climate change then use the excuse that people need help fighting climate change to justify more oil extraction to pay for the help.
The link to Prof. Wolff's article has changed to:
http://www.carbonbrief.org/guest-post-understanding-climate-feedbacks/
mods, if this violates the comment policy, I'm sorry but I'm just asking for a little help.
I'm an A-level student with a blog, and I posted something on 'science denial in school textbooks', along with an attempted (fairly brief) rebuttal to something I found in one of my textbooks. I'd really appreciate it if you guys had a look and check I haven't made a fool of myself :D
https://qedscience.wordpress.com/2015/10/12/science-denial-in-school-textbooks/
I've read from SkS for about 5ish(?) years now so I haven't just turned up to spam.
thanks for reading.
[RH] Activated link.
qedscience @4, technically questions like this should be posed on one of the Weekly Digest posts (where virtually all questions are on topic) rather than in other specific blogs where it is likely (and happens to be the case here) that they are not on topic. That said, I will briefly respond.
First, I consider it academic malpractise, equivalent in severity to plagiarism or fraud, to publish in text books work that you have not submitted and published in the peer reviewed literature. It always represents the strategy of the charlatan, of trying to persuade those without the relevant skills and expert knowledge to properly to assess your claims when you know full well that you are unable to persuade, and have not even tried to persuade, those with the relevant skills and expert knowledge. Where I a publisher, I would cease publication of all books authored by somebody who would stoop to this tactic.
Second, your response is quite good, but you do make some mistakes. In particular, volcanism was not a significant factor in build up of CO2 concentration from glacial peak to interglacial. In fact increased global temperature will increase CO2 concentrations, at least partly by reducing the ability of water to dissolve CO2. That effect probably results in a 10 ppmv increase in concentration per degree C increase in global temperature, although it may be as much as 20 ppmv per 1 degree increase. What happens when the world starts warming from a glacial is that the increased temperature results in an increase in CO2 concentration, which in turn results in a further increase in temperature.
Beyond that point, your response is very good.
Qed, I have posted a comment at your blog. The author of your text, George Facer, seems to exhibit a disturbing pattern of injecting his biased and ill informed personal views on climate change into his textbooks.
Thanks for this post. It is very informative.
One of the issues that it highlights, however, is the nature of climate change uncertainty within the whole debate. It also highlights why the climate models aren't perfect, a feature common to all modelling. Even the economic models used for microtrading where shares are automatically bought and sold on stock markets in milliseconds aren't perfect. Although the models do a good job, none provide a complete picture because certain compromises have to be made because not all feedback effects are understood as entirely as they need to be and the supercomputers used for the models do not have the capacity to deal with them all.
Dealing with feedback effects is one of the main reasons that climate models do not have the necessary resolution to make predictions as precisely as the naysayers would have the models do. Because the impact of all feedbacks and the chaotic nature of the phenomena being studied means that no single model has perfect resolution. It also means no model makes perfect predictions. That is the reason why naysayers are able to sow seeds of uncertainty, create confusion and peddle their propaganda for political purposes. However, it does seems that all the models have one thing in common, if CO2 increases are taken out of the equation then the models aren't very good at simulating the real world. It also means that naysayers should be taking all the models into account, not just the ones that suit their anti-AGW and CC propaganda.
As I undertsand it, Hansen, over a decade ago, asked for a satellite to be put into orbit to obtain data related to aerosols and cloud cover so he could further refine the climate models. This was seen as unnecessary by the Bush administration. I don't know if such satellites and others required to study the impacts of climate change feedbacks exist nowadays, however, the whole episode seems to indicate those prone to naysaying because of their political views and financial interests, don't see obtaining the data as necessary. They just don't seem to even want to know.
The trouble with uncertainty related to CC due to an incomplete knowledge related to feedbacks, however, is that the outcome can be either good or bad. There is uncertainty where other more certain indicators show that something favourable should still result. This means there is no need to worry other than gaining more understanding and knowledge. Or there is an uncertainty where other certain indicators show that something undersireable may happen. This means we do need to be worried, and we do need to gain more knowledge and understanding so we can develop strategies to deal with it. Our imperfect understanding of how CC feedbacks will ultimately impact the world's climate, does have uncertainty attached to it. However, there are other indicators where our knowledge is much more certain. None of our more certain indicators show that the CC outcomes that are likely to be desireable. This is uncertainty that we should be wary of, uncertainty that even the naysayers should be wary of, and it does mean that we need to further diminish the uncertainty related to feedbacks and tipping points through further study so we gain more knowledge and understanding. This should allow the resolution of the various CC models to be further refined.
QEDSscience @4. I posted a comment on your blog. That he makes the comments he does in a Chemistry textbook is astounding.
@5,6,8
I have responded to your comments on my blog, I have found them very useful and thank you all for commenting. You may wish to read all the responses as they interlink to a certain extent.
Tom Curtis @5
Thank you for that clarification on volcanoes. For that paragraph in general, I did address it in one of my responses. You are right in concluding it was overtly 'emotional', I'll try not to do that in future.
Jim Eager & Glenn Tamblyn @6,8
I agree that it is really a disgrace that he was allowed to make those kind of statements in a school textbook.