Tuesday, September 25, 2012

Refuting the Climate Skeptics' Hypothesis

Regular readers of the Mabinogogiblog (peace be upon them) will know that I am interested, nay, obsessed, with Climate Sensitivity, which is the extent to which the Earth's temperature changes in response to a given input, which is conventionally taken as a doubling of the level of CO2 in the atmosphere.

This is because in spite of everything (*glares in direction of free market fundamentalists*), I love humanity, especially our children and grandchildren, and they are going to have a hard time of it living in an environment that we have depleted and made hostile because we have released over the last 150 years CO2 that was locked up over the course of many millions of years.

The Green movement, and indeed, most of the world's leaders,  wants to see the world move to an energy system based on income - incoming solar energy - rather than by using up energy capital - fossil fuels - and toxic capital at that. This necessary transition is being frustrated by climate sceptics, who are wont to put an infinite number of obfuscatory questions about all aspects of climate change, but who are unwilling and unable or  to answer any questions about their own position.

It is time for the "sceptics" to answer one question: Does your hypothesis fit the facts?

The skeptics' hypothesis is that the CO2 changes we are making will not adversely affect our planet's climate.

The simplest knock-down to the skeptics' hypothesis is that if the climate sensitivity were zero or close to it, we should not have seen the natural variations in climate in the past that skeptics are so fond of emphasising. ("Climate has always changed. It's natural")
For instance, in the Ice Ages, after a relatively small change in solar warming, the global temperature changes by 6*C or more. Low climate sensitivity values cannot explain that change.

But there is more that they cannot explain.

This is the logic of anthropogenic (man-made) climate change:

1 The Greenhouse Effect exists. Without it, Earth temperature would be -15, not +15*C.
2 CO2 is a greenhouse gas.
3 CO2 concentrations have increased by 41% since the Industrial Revolution
4 That is certain to increase the planet's temperature
5 Positive feed-backs mean that the temperature will increase further, to dangerous levels.

1-4 are undeniable textbook physics.
Any questions about the basic physics? Read Judith Curry here.

Therefore the climate change contrarians' case depends entirely on challenging point 5.

The contrarian  hypothesis is that the feed-backs are non-existent or tiny.

So let's take a look at feed-backs - events that take place in response to any warming of the Earth.

Here is a list of the feedbacks that result from the warming that takes place.

Positive feedbacks
  1. Water vapour increases. Water vapour is the main greenhouse gas. 
  2. Ice melts, so its albedo (reflectivity) is diminished. This is happening currently in the Arctic. 
  3. Cloud feedback: this is highly complex and uncertain, but the best net measurement comes from Dessler, who finds that cloud feedback is mainly positive. As the planet warms, cloud cover decreases, so whatever the cloud effect, it will decrease in a warming climate.
  4. Methane releases from land (permafrost) and sea (clathrates). Recent work has identified 4 billion tonnes of methane that could be released from the sea. 
  5. CO2 releases from warming soil and oceans " there’s about twice as much carbon frozen into the permafrost as is in the atmosphere, "
  6. CO2 releases from increasing forest fires in drier, warmer climates
  7. Diminished CO2 absorption by warming oceans. About 50% of the CO2 that we have emitted is absorbed by the oceans.  
  8. Albedo changes from forest fires (blackened land will absorb more incoming radiation, but also emit more outgoing radiation).
  9. Vegetation spreading towards the North Pole.

Negative Feedbacks
  • Lapse rate feedback : Warm air rises, the top of the atmosphere warms, and heat radiates more efficiently into space.
  • Natural dusts blown from deserts have complex effects, possibly netting out as a weak negative feedback. 
  • Stefan-Boltzmann feedback - more heat will be radiated away from a warmer planet. This may be the limiting feedback that puts a stop on runaway global warming of 6*C+. 

Note that in this list there are nine positive feed-backs and three negative feedbacks.
In terms of quantity, a rough idea of the relative values of three main feedbacks can be gained from the IPCC 2007.

Here is a figure summarising the feedbacks used in computer models:

Figure 8.14. Comparison of GCM climate feedback parameters for 
water vapour (WV), cloud (C), surface albedo (A), lapse rate (LR) 
and the combined water vapour plus lapse rate (WV + LR) in units 
of W m–2 °C–1. ‘ALL’ represents the sum of all feedbacks. 

Results are taken from Colman (2003a; blue, black), Soden and 
Held (2006; red) and Winton (2006a; green). Closed blue and open 
black symbols from Colman (2003a) represent calculations 
determined using the partial radiative perturbation (PRP) and the 
radiative-convective method (RCM) approaches respectively. 
Crosses represent the water vapour feedback computed for each 
model from Soden and Held (2006) assuming no change in relative 
humidity. Vertical bars depict the estimated uncertainty in the 
calculation of the feedbacks from Soden and Held (2006).

Note that the final figure, which includes Water Vapour, Cloud and Lapse Rate (negative) feedback, is around 2 Watts per sq meter per *C rise in temperature, with error margins of 1.5-2.4 Watts per sq metre.

Note that these calculations take no account of CO2 and methane release feed-backs, which means that the models are probably underestimating the situation. A recent paper calculates that methane feed-backs may add 0.25-1.0*C to global temperatures in the next 100 years.

There are other feed-backs that may take place. For instance, conversion of forest to desert will have complex effects, with releases of CO2 from fire, initial decreases in albedo, increases in methane, followed by increases in albedo.

One objection of skeptics to the idea of positive feed-backs is that if they operated forever, the earth would have boiled up ages ago. This does not happen, since there are limits to the feed-backs. Ice albedo effect is  limited, since once all the ice has melted, the feedback ceases to operate. There is a discussion of runaway warming here.

In summary, consideration of the nature of feed-backs shows that positive feed-backs far outweigh negative. This  is incompatible with the climate skeptic hypothesis of zero or low effect from CO2 increase. 

The climate skeptics' hypothesis is therefore false.


Anonymous No.6 (pbum) said...

Excellent stuff Doc.

I salute you indefatigability. Quite an inspiration.

No. 6 (peace be upon me)

DocRichard said...

Dear pbum, thank you for your kind words. They are much appreciated. It made me realise today that I am not just writing for the sake of writing, but that some kind souls are actually taking the trouble to read this stuff. Which is really encouraging. Thank you.

DocRichard said...

Thanks pbum. An ounce of bouquets is better than a hundredweight of brickbats.

Roger Davies @4589roger said...

Excellently thought out and expressed. Clarified my mind! Thanks:)
One detail You say point 6 is the "sceptics'" only argument, but it's point 5 above that.

crandles said...

I see you have three bullet points for negative feedbacks but then say there are 9 positive and 2 negative.

Perhaps on the inconsistency you are right!

The natural dust seems from the article linked would appear to be a negative feedback on CO2 levels and it also affects regional temperatures from which I infer there is little effect on global mean temperature.

When it comes to calculating the climate sensitivity to CO2, this CO2 feedback does not play a part in the temperature result calculation. In a way it does play a part because it means that the amount of emissions needed to double the atmospheric level is increased.

That feedback is small beer (and I suggest often considered part of ocean uptake) compared to ocean (and land) uptake of CO2 which you have as no 7. That isn't a feedback but it means that the amount of emissions needed to double atmospheric level is about doubled.

So you correctly have three negative feedbacks listed but only two would play a part in the calculation of climate sensitivity to CO2.

Unfortunately we now need to go back to the list of feedbacks and see that No 4 relates to methane and is not a temperature rise in response to CO2. This does decompose to CO2 so like 5 and 6 potentially affects the level of emissions needed to get to a doubled CO2 level.

6 and 8 are possibly more about a timing differences as CO2 emitted in burning will be absorbed in regrowth but you can argue that the level of burnt land at any time will increase with temperature. So 8 is OK.

So by my count that leaves us with 1,2,3,8 and 9 ie only 5 positive feedbacks not 9 affecting the calculation of climate sensitivity to CO2.

Anyway, to improve the article I would split the feedbacks between those that affect the calculation of the result of climate sensitivity to CO2 and the others that don't.

Ocean and land uptake deserve mention but shouldn't be called a feedback. I would mention this has already happened so the 41% CO2 increase is after this has occurred.

crandles said...

You asked how to calculate sensitivity from models. The how is easy: You lease a model run at a prescribed CO2 level (typically preindustrial 280ppm) until the temperature levels out. Then run the model with double the level of CO2 until temperature levels out again. A full ocean model might and should take about 1000 years. There are ways to speed this up by running the ocean only for say 800 of those 1000 years. This how is easy. The question of which model(s) with which parameters is complex even if the idea of using the one(s) which best match historical data is easy to state.

Calculating from climate data is more difficult. Using recent temperature rise gives an idea but it is rather uncertain. We have seen 0.8C of warming. Because of thermal inertia of oceans we are likely to see at least 0.5C more warming if CO2 held at current levels. Solar irradiance has not increased over last 50 years and volcanoes tend to have fairly short lived effect so the scientist reckon than natural effects would have caused a very slight cooling while methane levels have risen. So if we ignore these other effects we happen not to be miles out.

Co2 has risen 41% from preindustrial and square root of 2=1.41 so this 41% represents half a doubling. So (0.8+0.5)*2 =2.6C is a rough estimate.

The problem with this is that the 0.5C further warming if CO2 held at current levels is uncertain and could easily be quite a bit more. That number can only really come from models.

Better constraints come from things like temperature response to volcanoes and change since last glacial maximum. Again this needs models and it is a matter of comparing what has happened to what models with different climate sensitivities say should happen.

I don't know if that helps at all.

DocRichard said...

Hi Crandles

Many thanks for these helpful comments. Sorry I have been too busy with the projections posts to come back to this.

I am deeply impressed with your sq root of 2 thing. Mindblowing simplicity.

But we need to get this thing about climate sensitivity far better understood, to cut through the fog of misunderstanding put out by the contrarians.

crandles said...

James Annan has a couple of posts on his blog about using the last glacial maximum to constrain climate sensitivity. Thought you might want to have a read: