The gist of the highlights of the annual update of the global carbon budget and trends of the Global Carbon Project reads: Anthropogenic CO₂ emissions have been growing about four times faster since 2000 than during the previous decade. Natural CO₂ sinks are growing, but more slowly than atmospheric CO₂, which has been growing at 2 ppm per year since 2000. This is 33% faster than during the previous 20 years. All of these changes characterize a carbon cycle that is generating stronger climate forcing and sooner than expected.

CO2 emission growth four times faster, atmospheric CO₂ growing 33% faster, generating stronger climate forcing and sooner than expected. This makes a nice headline - or rather a scary one. Climatepatrol attempts to dig deeper beyond the news releases on global carbon trends 2007.  The papers and posts are many which deal with the relationship between global temperature history, carbon cycle and IPCC emission scenarios. I try to sort out some of the newer, controversal papers and divide them into three groups:

A) Constraining temperature history and past emissions in climate models

Stephen E. Schwartz about the huge uncertainties of aerosols and about HEAT CAPACITY, TIME CONSTANT, AND SENSITIVITY OF EARTH’S CLIMATE SYSTEM (Schwartz 2007) and the answer by the prominent team G. Foster, J. Annan, G. Schmidt (Realclimate), M. E. Mann (Realclimate, Hockey Stick);  The consequences of not knowing low- and high-latitude climate sensitivity, D. Rind (2008); Shaviv’s blog, 2006″ on “Cosmic Rays and Climate” (sciencebits); cosmic rays and cloud project of C.E.R.N, Geneva, Switzerland; numerous publications by Henrik Svensmark about cosmic rays, clouds, aerosols; Scafetta, West (2007) on solar forcing; Josh Willis, JPL Nasa, as interviewed by Climate Science Weblog defends the IPCC position regarding human induced ocean warming rather vaguely:

Estimates like those from Levitus are good enough to say that the Earth has been out of radiative balance for much of the last 50 years, but they are not accurate enough to get the interannual fluctuations right, and the decadal variations were plagued by data problems. As we fix these problems, we will get a better idea of what these interannual to decadal changes look like and we can begin to ask if the models are getting it right.

B) Relationship between temperature history and effective climate sensitivity

Constraints on the transient climate response from observed global temperature and ocean heat uptake, R. Knutti, L. Tomassini, ETH Zurich (AGU 2008); Climate Forcing and Climate Response, IPCC TAR, the scientific basis, chapter 9.2.1; Monckton and Climate sensitivity - teacher or student?

C) Relationship between effective climate sensitivity and emission scenarios

Constraining Climate Model Parameters from Observed 20th Century Changes, Chris E. Forest, Peter H. Stone and Andrei P. Sokolov, MIT Joint Program on the Science and Policy of Global Change, April 2008; and here is a quote from the abstract of a simplified version of a paper entitled “Chaotic Radiative Forcing, Feedback Stripes, and the Overestimation of Climate Sensitivity”, submitted on June 25, 2008 for publication in the Bulletin of the American Meteorological Society. Author: Roy W. Spencer, Ph.D.:

The new interpretation suggests a very low sensitivity. If the new sensitivity estimate is accurate, it would suggest only 0.5 deg. C of manmade warming by the year 2100. The new sensitivity estimate also suggests that warming over the last century can not be explained by human greenhouse gas emissions alone, but instead might require a mostly natural explanation.

The IPCC position

Chapter 3 of the Special Report on Emission Scenarios (IPCC TAR 2001) devides the driving forces for future emission and temperature scenarios until 2100 into six categories :
1) Population
2) Economic and Social Development
3) Energy and Technology
4) Agriculture and Land-Use Emissions
5) Other Gas Emissions
6) Policies
Climatepatrol finally found the effective climate sensitivity as used in the AOGCM model runs. It is 2.5°C for an immediate doubling of greenhouse gases compared to preindustrial levels.

The findings of the recent research and the IPCC position

Supposing, the leading scientists Schwartz, Shaviv, Scafetta, Spencer - only to name a few with recent papers - are wrong when suggesting a low sensitivity of the climate towards human influences, we first look at the analysis of the climate sensitivity as used in the AOGCMs. Note: AOGCMs produce a range of temperature response to SRES emission scenarios. In their MIT joint program on the Science and Policy of Global Change {(see C) above}, Forest, Stone, Sokolov (2008), came up with a climate sensitivity estimate which they calculated as being used for the model runs in the various SRES-scenarios when making 21st century temperature projections. The transient response of the models as evaluated by the new MIT model is equivalent to an effective climate sensitivity of approx. 2.7 K in average for a doubling of CO₂, slightly above the 2.5 K sensitivity as set in the AOGCM models and as used for the SRES scenarios in IPCC TAR. But here comes the real problem: Far too much heat was projected by AOGCM models as being taken up by the oceans when compared with 20th century measurements of ocean heat content according to the new study. The slightly higher effective climate sensitivity as a result of a slower heat transfer to the oceans [and matching it with equilibrium sensitivity - sic!] can still allow the models to reproduce the 20th century warming. They simply have to take use of the uncertainty level of the influence of aerosols in order to have the models reproduce a stronger cooling, especially before CO₂ -concentration in the atmosphere started to accumulate faster throughout the last decades. According to the models, the influence of aerosols is then projected to diminish as CO₂  continues to rise in the 21st century.

Climatepatrol’s two cents’ worth

So the models are constrained in a way, e.g. by assuming large aerosol cooling, to reproduce the 20th century warming,  which is just +0.6°C until 1990 compared to a projected further warming of 1.1 - 6.4°C until 2100 (IPCC range), depending on further emissions. This high uncertainty range for future temperature indirectly suggests to the policy makers that we can make all that difference. Can we really organise such a change in temperature? As far as the emissions are concerned, China has just overtaken the U.S. with their energy intensive economic boom along with its energy intensive cement and steel.

 The difference we can make using effective SRES climate sensitivity of 2.7 till 2100

The transient climate sensitivity for a doubling of CO₂  in the atmosphere assumes an increase of CO₂  concentration in the atmosphere of 1% per annum. This will result in an atmospheric CO₂ -concentration of 964 ppm in 2100. In order to get there, we could even start to emit 17 gt of Carbon a year (2007 = 10 gt) and end up emitting more than 30 gt in 2100, provided the airborne fraction will not exceed 55% (while still 45% are captured by land and ocean by 2100). The airborne fraction as reported by the Global Carbon Project went up from 40% to 45% during the past 50 years, yet with strong interannual fluctuations. Temperature increase for such an extreme event would be about 3.6°C until 2100 above 1990 values. Ironically, the CO₂ in the atmosphere increases on a much slower rate at the time being, contrary to what the headlines from the global carbon trends may suggest. Now, instead of using an average of roughly 25 gt of carbon (which is unlikely anyway given the limited ressources of fossil fuels), lets assume we maintain emissions at say 15 gt (21st century average), the effect on our temperature will be about -1.1°C in 2100 (with effective climate sensitivity assumed to be 2.7 K) to end up at around +2.5°C in 2100. However, this projected warming may be further reduced to a few tenths of a degree because of the mitigation effect of reduced weight of aerosols in the atmosphere compared to a contineously increasing CO₂-concentration. Many scientists point out this effect of mitigation on the immediate temperature response of the climate system. And we were talking about stabilizing emissions here, by which we end up being below the SRES-scenarios of the A-families (e.g. A2). Having said this, same effect means that by quickly reducing emissions, CO₂ will continue to accumulate in the atmosphere, but the cooling aerosols will go down immediately. The possible result: No measurable effect on global temperature by 2050 but possible shifts in regional climates because of reduced aerosols, more droughts, more famines, more poverty. It’s possible. We simply don’t know.

The difference we can make assuming a lower climate sensitivity

Now if the above listed climate skeptics are right and climate sensitivity is at the lower end or even below the IPCC range, global warming should no more be an issue, mitigation of global warming should not. Let’s concentrate then on the real challenges of this century: Clean water, food, natural resources, health, freedom and peace, and of course research and development of clean energy resources and adaption which may include reforestation, dams, dikes, etc. Energy rationing will first of all affect the poor. The development of clean energy sources rather than mitigation of the climate is the key. It was pointed out during the “Copenhagen consensus” that, provided that climate sensitivity is low, any mitigation efforts designed to reduce emissions, with an aim of lowering global surface temperature, would harm human welfare more than its benefits would be. This is true from a mere economical point of view as discussed by leading economists (The Copenhagen Consensus). Naturalists, please don’t bash the economists for not understanding the climate system, because the punch line is what the IPCC lead authors have to say about climate prediction: Among many other issues, they found “serious inadequacies in climate change prediction that are of real concern”. (By Environmental Research Web and found here)

Notes:

“Equilibrium climate sensitivity”, as commonly used in the IPCC, is the sensitivity that translates to a certain increase in average global surface air temperature per unit of forcing. For a doubling of CO₂, the new forcing results in an increase of about 1.2°C, yet without taking into account feedbacks of the earth system as a result of this new forcing ; IPCC, chapter 9.

“Transient climate sensitivity” for 1%/yr increasing CO₂
A common standardised forcing scenario specifies atmospheric CO₂ to increase at a rate of 1%/year compound until the concentration doubles (or quadruples) and is then held constant. The CO₂ content of the atmosphere has not, and likely will not, increase at this rate (let alone suddenly remain constant at twice or four times an initial value). If regarded as a proxy for all greenhouse gases, however, an ?equivalent CO₂? increase of 1%/yr does give a forcing within the range of the SRES scenarios. (IPCC TAR, the scientific basis, chapter 9, transient climate response TCR).

“Effective climate sensitivity”  is a measure of the strength of the feedbacks at a particular time and it may vary with forcing history and climate state (IPCC TAR, chapter 9).