Arguments
Clouds are the only plausible feedback that could significantly dampen future global warming. Photograph: Graham Turner for the Guardian
What you need to know about climate sensitivity
Posted on 10 May 2013 by dana1981
This is a partial re-post of my latest in the Guardian's Climate Consensus – the 97%. It's intended as a basic primer to reference the next time somebody tells you global warming is nothing to worry about because climate sensitivity is low.
What you need to know about climate sensitivity
Climate sensitivity is a subject sometimes explored in mainstream media articles. For example, The Economist tried to summarize some recent research on the subject, although as climate scientist Michael Mann and I noted in an article for ABC, they made some key mistakes.
What is climate sensitivity?
We know that humans have increased the greenhouse effect due to the carbon emissions associated with burning fossil fuels. This increased carbon dioxide traps more heat in the Earth's atmosphere, causing a global energy imbalance. There is more energy incoming than escaping, and as a result the planet will warm until it reaches a new balanced energy state (equilibrium), with equal incoming and outgoing energy.
We also know that if we double the amount of carbon dioxide in the atmosphere, the increased greenhouse effect will cause the planet's average surface temperature to warm about 1.2°C (2.2°F) in response. That may not sound like very much, but the difference between an ice age and the current warm period is only about 5°C (9°F). Seemingly small temperature changes make a big difference in the Earth's climate.
In addition, there are feedbacks that can dampen or amplify the warming from the increased greenhouse effect. For example, when ice melts it makes the Earth's surface less reflective, causing it to absorb more sunlight and warm further. A warmer atmosphere will also hold more water vapor, and water vapor is another greenhouse gas.
The term "equilibrium climate sensitivity" refers to the total amount of warming that will occur at the Earth's surface once it reaches a new balanced energy state, including from the increased greenhouse effect from a doubling of atmospheric carbon dioxide, and including these feedback effects.
What are the biggest feedbacks?
Water vapor is probably the largest individual feedback. A 2009 study published in the prestigious journal Science by Andrew Dessler and Steven Sherwood found that, as climate scientists expected, the amount of water vapor in the atmosphere is increasing by enough to double the warming from the increased greenhouse effect of carbon dioxide.
As discussed above, melting ice is another significant warming feedback. Releases of stored carbon from beneath permafrost and methane from the deep ocean would also amplify global warming.
Unfortunately we don't know of many large negative feedbacks that would dampen global warming. Global plant growth has increased so far, and plants absorb carbon dioxide. But that trend probably won't last as extreme weather events like heat waves and droughts that damage plant life become more common.
Clouds are really the only plausible feedback that could significantly dampen future global warming. They're tricky because clouds cause both warming by increasing the greenhouse effect, and cooling by reflecting sunlight away from the Earth's surface. High clouds tend to have an overall warming effect, while low clouds tend to have an overall cooling effect.
So what types of clouds will become more abundant in a warming world, and what will the net effect on temperatures be?
Nice summary Dana, thanks.
John Mason just alerted SkS to a new paper in Science by Brigham-Grette et al. (2013). They examined sediment cores from a lake in Siberia going back over 3 million years. Their findings are not reassuring and suggest that fast-feebacks (and by extension equilibrium climate sensitivity) may be more aggressive than previously thought:
"Evidence from Lake El’gygytgyn, NE Arctic Russia, shows that 3.6-3.4 million years ago, summer temperatures were ~8°C warmer than today when pCO2 was ~400 ppm. Multiproxy evidence suggests extreme warmth and polar amplification during the middle Pliocene, sudden stepped cooling events during the Pliocene-Pleistocene transition, and warmer than present Arctic summers until ~2.2 Ma, after the onset of Northern Hemispheric glaciation."
These findings suggest that we are very likely in for a whole lot of hurt in the coming decades. Even more worrying is that we are already at 400 ppmv-- I shudder to think what the consequences for future generations will be should we continue with BAU and reach over 1000 ppmv...
The results of the study that Albatross cites above are also summarized in:
New study tells three million-year old story of the Arctic by Roz Pidcock, The Carbon Brief, May 9, 2013
Thanks very much for a good article. If warming due to CO2 doubling is 1.2 degrees and most models have lower sensitivity values of 1.5 degrees with means of around 3 C presumably the models all assume some overall positive feedback. How much uncertainty is there about cloud feedback?
Many years ago I had a lecturer in Mechanical Engineering who described the first building that had a comprehensive centrally controlled air conditioning system that he had designed. Complaints from the new occupants were thick and fast. Rooms were 'too hot' or 'too cold'. His solution was to put a 'thermostat' that people could adjust in every room. These 'thermostats' were not connected to the system! All complaints stopped!
Control is either illusory or real. Sometimes the illusion is more real.
How long will people ignore the realities? I am afraid that when only New York is flooded it may happen.
Bert
Peter Gleick has an excellent post here about the Pliocene climate with a lot of links.
scienceblogs.com/significantfigures/index.php/2013/05/10/the-last-time-atmospheric-co2-was-at-400-parts-per-million-humans-didnt-exist/
Please, help me understand some points on Climate Sensitivity. Does it refer to doubling of CO2 concentration from preindustrial times, the doubling from the present 400 ppm or for any doubling of CO2 concentration? After how much time after the doubling will the temperature reach the value idicated by CS? Does CS depend on how fast the CO2 concentration raises? And will the temperature after reaching the CS target remain stable or rise further? Where can I find the exact definition of CS?
Erik
Re. eschwarzbach @ 6, as Andy Lacis said, "In the Hansen et al. (1988, JGR, 93, 9341–9364) paper we derived the logarithmic type formula
F(X) = log [ 1 + 1.2 X + 0.005 X2 + 0.0000014 X3 ]
where X is the atmospheric CO2 concentration in ppm. This formula gives the radiative forcing in terms of equivalent temperature change rather than in W/m2. Thus, for 350 ppm, F(350) = 7.0 °C (or 7.0/33.0 = 21.2% of the total (emphasis added_ terrestrial greenhouse effect), while for doubled CO2 (emphasis added) the formula yields ΔTo = F(700) – F(350) = 1.23 °C". That quote appears as a comment in http://chriscolose.wordpress.com/2010/08/27/adding-up-the-greenhouse-effect-attributing-the-contributions/. It should take care of your first question. Re the others, Climate sensitivity usually refers to the equilibrium state. It takes a long time to reach that state because of the Ocean's huge thermal inertia. How long? Close to a thousand years, if slow feedbacks are taken on board. Finally, the CS does not depend onthe rate at which you add CO2 to the air. The article by Colose is a good place to understand the subject.
Sam martin @3 - sensitivity isn't assumed, it's an output of climate models based on their simulations of the Earth's climate. And of course estimates from past climate changes (paleoclimate) are based on observations, not models. I'm not sure about the exact cloud feedback uncertainty, but it's pretty large.
eschwarzbach @6 - equilibrium climate sensitivity refers to the warming in response to any CO2 doubling on century timescales. In reality the exact amount of warming will differ at different temperatures, because some different feedbacks will probably apply, but it's a pretty good approximation.
Most equilibrium warming occurs within several decades. About two-thirds happens almost immediately (the transient response). Within a century after reaching a stable equilibrium state, just about all equilibrium warming should have occurred. However, there is also Earth System Sensitivity (in the ballpark of 6–8°C for doubled CO2), which acts on millennial timescales. This is due to very slow feedbacks, mainly large bodies of ice that take a very long time to melt.
If you scroll over the words climate sensitivity, our glossary will pop up and give you the IPCC definition of the term.
eschwarzbach @ 6 "Global Warming" is an increase in the ocean heat content. I suggest a lay person will have endless difficulty contemplating land-air & ocean surface temperature unless they start by understanding it's just heat moving through into ocean, has 97.5%, freshwater=2.3%, land+air=0.2% of ecosystem heat content. Land solid has no currents so thin veneer only involved (I read ~6.5m deep somewhere), ocean liquid so heat mixes ~6km deep (ex. trenches) ~4km avge, but over centuries-to-millenia. Equator to poles top veneer takes heat, warms poles, saltier, drops, returns at depth. Tiny bits of ocean heat like ENSO sucking in & burping out seem really big to us on/in the negligible 0.2% land+air bit. Surface temperature is (1) a proxy (2) a symptom (3) what determines heat in/out balance at TOA. Example: if ocean mixing slowed to stop for few years (just fantasy for example) land+air reaches TOA balance quickly at "final" temp & AGW pauses. Ocean mixing resumes in (unrealistic) example, surface temp drops & AGW resumes. Humans now see "global cooling" when it's the exact opposite - AGW now resumed. Exaggerated example shows why must understand ocean mass, depth, heat capacity (572 yrs for +1C @ +0.9 wm**-2 is what I computed), currents before can hope to understand surface temperature changes.
Should add @10 "thin veneer" for surface temp includes top ocean veneer. Need to study to decide a reasonable few metres depth in which thermal conductivity & local mixing significant. It's most significant because each ~6.5m ocean depth = all land+air heat. Land is significant for radiation balance only, not heat content, because it's mostly quite dark albedo.
eschwarzbach @ 6 Another way to view situation when "CS target remain stable". If, say, +2.8 degrees required to balance TOA in/out, I see no logical reason why 80% lowest ocean, now at ~6 degrees avge, will not need to be ~9 degrees avge before all is stable in centuries/millenia.
Dana. The comments policy on your Guardian articles is so draconian - or one of the moderators is so over the top - that it seems impossible to have a robust intelligent discussion at all. You may recognise my user name and, if so you may recall that I am not one for wild offensive tirades against climate "sceptics" yet my comments are being consistently deleted, as are those of many others. What gives?
dana1981 @ 9:
Yikes! That's the first I've heard of ESS and its 6–8°C. I have been stuck in my own little world, thinking the 3°C bandied about would be the end of the matter (barring methane amplification, which I didn't think was quantified as yet). Big wake-up call to whoever is asleep at the wheel. Thanks for cheering me up. Not.
Nick - perhaps you are responding to a particular comment that has been deleted? Any replies on that chain are likely to be deleted too.
Rob. I'm pretty sure I was initiating a comment thread...