Regular readers of the Mabinogogiblog (peace be upon them) will know that I am interested, nay, obsessed, with Climate Sensitivity.
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 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. 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.
- Water vapour increases. Water vapour is the main greenhouse gas.
- Ice melts, so its albedo (reflectivity) is diminished. This is happening currently in the Arctic.
- 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.
- Methane releases from land (permafrost) and sea (clathrates). Recent work has identified 4 billion tonnes of methane that could be released from the sea.
- CO2 releases from warming soil and oceans " there’s about twice as much carbon frozen into the permafrost as is in the atmosphere, "
- CO2 releases from increasing forest fires in drier, warmer climates
- Diminished CO2 absorption by warming oceans. About 50% of the CO2 that we have emitted is absorbed by the oceans.
- Albedo changes from forest fires (blackened land will absorb more incoming radiation, but also emit more outgoing radiation).
- Vegetation spreading towards the North Pole.
- 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.