[Published on The Drum 20 May]
Recently climate sceptics Anthony Cox and David Stockwell published an opinion piece on the ABC’s The Drum claiming climate scientist James Hansen had “admitted” climate models have been wrong, and that human-caused global warming was therefore in doubt. The article contains basic misrepresentations of Hansen and of the substance and implications of a draft paper by Hansen.
The Hansen paper does not weaken the case that humans are the main cause of global warming. On the contrary, it suggests we have unwittingly and temporarily shielded ourselves from the full effects of our activities.
Professor James Hansen, of the NASA Goddard Institute for Space Studies, recently (18 April) posted a draft scientific paper (pdf, 1.5 Mb) for comment. The paper argues that most computer models of climate have over-estimated the rate at which heat is being absorbed by the oceans. Because there is a trade-off in the models between heat entering the oceans and heat being reflected back into space by aerosols in the atmosphere, the implication is that the cooling effect of aerosols has been under-estimated.
This is not good news, because it implies aerosols have been partially shielding us from the full warming effects of carbon dioxide emissions. As and when aerosol pollution is cleaned up, global warming will get worse.
The Cox-Stockwell commentary misrepresents Hansen’s role and his intention in posting the draft paper. It uses prejudicial language. It claims Hansen merely assumed the warming effect of greenhouse gases is known, when in fact Hansen has clearly spelt out the independent evidence for this.
The commentary begins by claiming Hansen “published” his draft paper “free from the restraints of peer review” on a personal web site, implying that Hansen was engaging in a sly trick. In fact Hansen was quite clear that the paper was a draft and that critical comment was welcome. Posting is not at all the same as publishing.
The commentary characterises Hansen as “the world’s most prominent expert on the use of computer models for understanding of the Earth’s climate”, which gives the impression that Hansen is a great believer in the accuracy of computer models. In fact Hansen’s group is not one of the main computer modelling groups, and Hansen is quite clear, in his book Storms of my Grandchildren, that computer models are a useful tool to improve understanding but are not to be exclusively relied upon.
Cox and Stockwell repeatedly characterise the Hansen paper as a “frank admission” and a “concession” that climate models have had a significant inaccuracy. In fact Hansen is just doing normal science. He is trying to improve our understanding of climate and to improve the accuracy of climate models. All climate modellers know there are inaccuracies and poorly-constrained factors in the models. There has been no conspiracy to exaggerate the models’ accuracy and there is nothing to be “admitted”. Nor is there any implication that the case for human-caused global warming is weakened (more on that later).
Cox and Stockwell erroneously characterise a principal conclusion of the Hansen paper when they say that the models’ inaccuracy “has the effect of exaggerating the net AGW [anthropogenic global warming] in models”. AGW refers to the rise in temperature (the warming), and there is in fact no implication that the models have exaggerated the projected rise in temperature.
Rather, the implication stated by Hansen is that, in the models, “the corresponding net human-made climate forcing is unrealistically large” [emphasis added]. The forcing refers to the extra energy trapped in the atmosphere, not to the resulting temperature rise. The two major human contributions to the energy budget are from carbon dioxide, which is positive and accurately known, and from aerosols, which is negative and not accurately known, as Hansen has been saying for decades. In the models too much heat energy has been absorbed by the oceans, so not much aerosol cooling was implied. Hansen argues that in reality less heat is going into the oceans so the aerosol cooling must be larger and the net effect (carbon dioxide warming effect minus aerosol cooling effect) smaller.
Mainstream scientists and sceptics alike have been aware that the aerosol effect is complex and not accurately known. Hansen has for decades advocated a dedicated satellite to measure the aerosol effects, but this was not attempted until the past year, and the launch failed. Lacking direct measurements, the magnitude of the aerosol effect has been inferred after other energy terms have been accounted for. The total energy is implicitly constrained by requiring models to match the observed temperature record of the past century.
It is a widespread misconception, evidently shared by Cox and Stockwell, that the case for human-caused global warming rests mainly on computer models and is therefore vulnerable to the aerosol uncertainty. This misconception is part of the disinformation put about by the professional deniers funded by the likes of ExxonMobil.
In fact Hansen’s main argument is based on paleoclimate records, records of past climate variations extracted from ice and rocks. Cox and Stockwell would know this if they had properly read Hansen’s book, and a more recent paper also listed on Hansen’s web site (Paleoclimate Implications for Human-Made Climate Change, January 18).
During the ice ages the global mean temperature rose and fell by about 5 degrees Celsius every hundred thousand years or so. The measured variations of temperature and atmospheric carbon dioxide during the ice ages accurately constrain the so-called climate sensitivity to 3 degrees per doubling of carbon dioxide, plus or minus about 0.3 degrees. Because this number is based on a natural experiment, it accounts for many complicated subsidiary effects and feedbacks, such as the changes in water vapour and methane content in the atmosphere and changes in cloud cover, that accompany changes in temperature.
A side issue here is another widespread misconception, that carbon dioxide cannot be causing global warming because during the ice ages temperature changes occurred several hundred years before carbon dioxide changed. The reason for this is well understood, if not widely appreciated outside the climate science community. During the ice ages the trigger for temperature changes was small changes in the heat received from the sun. On their own these would have caused less than a degree or so of temperature change, but temperature rises cause carbon dioxide to be released from the ocean and soils, and this greatly magnifies the effect on temperatures. On the other hand the present warming is being triggered by human releases of carbon dioxide, not by variations in the sun’s heat, so the carbon dioxide rise is leading the temperature rise, rather than lagging behind as it did in the ice ages.
Thus Cox and Stockwell are incorrect when they claim “the effects of CO2 and feedbacks are also unmeasured”, and go on to propose that “another way to bring the energy into balance might be to reduce the presumed warming effect of greenhouse gasses”. Rather, the warming due to greenhouse gases (carbon dioxide, methane, etc.) is well constrained by paleoclimate, satellite and laboratory measurements (not all of which I have recounted here), and the net energy forcing is constrained by the observed temperature record of the past century, which includes the significant warming of the past few decades. The conclusion that human emissions are causing most of the recent warming remains strong.
The other recent paper of Hansen’s mentioned above, on paleoclimate, gives even more reason to be very concerned about our present situation. Hansen notes that a couple of the past interglacial periods were very close to the present temperature (which has been raised about 0.6 degrees by human causes) and that sea level was then several meters higher than at present. This implies that if the present temperature were maintained we would suffer a serious sea level rise, probably over the next century or so.
Of course the temperature will rise by several degrees more if we do not quickly change our ways. Paleoclimate records from 3 million years ago show that the temperature was only 1-2 degrees Celsius higher and sea level was around 25 meters higher than recent pre-industrial levels. A sea level rise of this magnitude, occurring at rates of meters per century, as observed in paleoclimate records, would destroy many coastal cities and much low-lying land. It would be very difficult to rebuild and maintain port infrastructure. Along with major changes in climatic zones, storms, vegetation, and food and water supplies, it is hard to see how our industrial civilisation could survive in anything like its present, globally connected form.
Thus Hansen’s recent work warns us that we seem to be much close than previously realised to a threshold above which global warming and sea level rise will run out of control, and the world will tip into a quite different climate regime. This is why he argues that the temperature rise should be limited to only 1 degree more, not the 2 degrees commonly assumed by politicians, and brought down again as quickly as possible. This requires the atmospheric carbon dioxide to be brought down as quickly as possible from the present 390 parts per million to no more than 350 parts per million. If aerosols have been limiting the warming we are causing, then the task is even more urgent.
The writings of climate “sceptics” typically betray a patchy and incomplete understanding of climate science, and the Cox and Stockwell commentary is no exception. This is symptomatic of a tendency to cherry-pick the science for uncertainties they hope will discredit the main conclusions. Their poor scholarship does our society a grave disservice.