On December 4 2011 there was an article in Nature by Quirin Schiermeier that claims a quantification of the human influence on global warming. The article is
Three-quarters of climate change is man-made. NatureDOI:doi:10.1038/nature.2011.9538. Nature Geoscience Year published:(2011)DOI:doi:10.1038/ngeo1327
with the title
Independent study quantifies human influence on global warming
Excerpts from the article read [highlight added]
Natural climate variability is extremely unlikely to have contributed more than about one-quarter of the temperature rise observed in the past 60 years, reports a pair of Swiss climate modellers in a paper published online today. Most of the observed warming — at least 74 % — is almost certainly due to human activity, they write in Nature Geoscience.
Since 1950, the average global surface air temperature has increased by more than 0.5 °C. To separate human and natural causes of warming, the researchers analysed changes in the balance of heat energy entering and leaving Earth — a new ‘attribution’ method for understanding the physical causes of climate change
Their findings, which are strikingly similar to results produced by other attribution methods, provide an alternative line of evidence that greenhouse gases, and in particular carbon dioxide, are by far the main culprit of recent global warming. The massive increase of atmospheric CO2 concentrations since pre-industrial times would, in fact, have caused substantially more surface warming were it not for the cooling effects of atmospheric aerosols such as black carbon, they report.
Knutti and his co-author Markus Huber, also at ETH Zurich……. utilized a much simpler model of Earth’s total energy budget and ran the model many thousands of times, using different combinations of a few crucial parameters that contribute to the energy budget. These included global values for incoming shortwave radiation from the Sun, solar energy leaving Earth, heat absorbed by the oceans and climate-feedback effects (such as reduced snow cover, which amplifies warming by exposing darker surfaces that absorb more heat).
By using the combinations that best matched the observed surface warming and ocean heat uptake, the authors then ran the so-constrained model with each energy parameter individually. This enabled them to estimate the contribution of CO2 and other climate-change agents to the observed temperature change.
Knutti and Huber found that greenhouse gases contributed 0.6–1.1 °C to the warming observed since the mid-twentieth century, with the most statistically likely value being a contribution of about 0.85 °C. Around half of that contribution from greenhouse gases — 0.45 °C — was offset by the cooling effects of aerosols. These directly influence Earth’s climate by scattering light; they also have indirect climate effects through their interactions with clouds.
The authors calculated a net warming value of around 0.5 °C since the 1950s, which is very close to the actual temperature rise of 0.55 °C observed over that period. Changes in solar radiation — a hypothesis for global warming proffered by many climate sceptics — contributed no more than around 0.07 °C to the recent warming, the study finds.
To test whether recent warming might just be down to a random swing in Earth’s unstable climate — another theory favoured by sceptics — Knutti and Huber conducted a series of control runs of different climate models without including the effects of the energy-budget parameters. But even if climate variability were three times greater than that estimated by state-of-the-art models, it is extremely unlikely to have produced a warming trend as pronounced as that observed in the real world, they found.
“This tightens estimates of past responses,” says Gabriele Hegerl, a climate scientist at the University of Edinburgh, UK, “And it should also lead to predictions of future climate change that are grounded in the kind of changes already being observed.”
Quirin Schiermeier bases his text on the papers
D. M. Murphy, S. Solomon, R. W. Portmann, K. H. Rosenlof, P. M. Forster, and T. Wong (2009), An observationally based energy balance for the Earth since 1950, J. Geophys. Res., 114, D17107, doi:10.1029/2009JD012105.
with the abstract
We examine the Earth’s energy balance since 1950, identifying results that can be obtained without using global climate models. Important terms that can be constrained using only measurements and radiative transfer models are ocean heat content, radiative forcing by long‐lived trace gases, and radiative forcing from volcanic eruptions. We explicitly consider the emission of energy by a warming Earth by using correlations between surface temperature and satellite radiant flux data and show that this term is already quite significant. About 20% of the integrated positive forcing by greenhouse gases and solar radiation since 1950 has been radiated to space. Only about 10% of the positive forcing (about 1/3 of the net forcing) has gone into heating the Earth, almost all into the oceans. About 20% of the positive forcing has been balanced by volcanic aerosols, and the remaining 50% is mainly attributable to tropospheric aerosols. After accounting for the measured terms, the residual forcing between 1970 and 2000 due to direct and indirect forcing by aerosols as well as semidirect forcing from greenhouse gases and any unknown mechanism can be estimated as −1.1 ± 0.4 W m−2 (1σ). This is consistent with the Intergovernmental Panel on Climate Change’s best estimates but rules out very large negative forcings from aerosol indirect effects. Further, the data imply an increase from the 1950s to the 1980s followed by constant or slightly declining aerosol forcing into the 1990s, consistent with estimates of trends in global sulfate emissions. An apparent increase in residual forcing in the late 1990s is discussed.
and the new paper
Markus Huber and Reto Knutti, 2011: Anthropogenic and natural warming inferred from changes in Earth’s energy balance. Nature Geoscience doi:10.1038/ngeo1327
with the abstract
The Earth’s energy balance is key to understanding climate and climate variations that are caused by natural and anthropogenic changes in the atmospheric composition. Despite abundant observational evidence for changes in the energy balance over the past decades, the formal detection of climate warming and its attribution to human influence has so far relied mostly on the difference between spatio-temporal warming patterns of natural and anthropogenic origin. Here we present an alternative attribution method that relies on the principle of conservation of energy, without assumptions about spatial warming patterns. Based on a massive ensemble of simulations with an intermediate-complexity climate model we demonstrate that known changes in the global energy balance and in radiative forcing tightly constrain the magnitude of anthropogenic warming. We find that since the mid-twentieth century, greenhouse gases contributed 0.85 °C of warming (5–95% uncertainty: 0.6–1.1 °C), about half of which was offset by the cooling effects of aerosols, with a total observed change in global temperature of about 0.56 °C. The observed trends are extremely unlikely (<5%) to be caused by internal variability, even if current models were found to strongly underestimate it. Our method is complementary to optimal fingerprinting attribution and produces fully consistent results, thus suggesting an even higher confidence that human-induced causes dominate the observed warming.
There are several problems, however, with their conclusions:
1. The Huber and Knutti study is “[b]ased on a massive ensemble of simulations with an intermediate-complexity climate model“. The appeal to the Murphy et al 2009 study, while it does not use a global climate model still uses “radiative transfer models“. It is a circular argument to use models to segment radiative forcings when the models do not have all of the climate forcings and feedbacks correctly represented (NRC, 2005).
2. The recent lower magnitude of global warming (e.g. see upper ocean content, and see lower troposphere – figure 7 top) does not fit into their conclusion, and, moreover, suggests natural multi-year (and perhaps mutlti-decadal) variations in radiative forcing are larger than they concluded from their modeling based study. If the heat is going deeper into the ocean, it is, therefore, not even being considered in their analyses yet this is part of global warming, as discussed in my post
3. There is no way that human climate forcings from aerosols are as well understood as claimed in these studies; eg. see for example my post
and see the 2010 American Meteorological Statement on
which includes the finding that
By partially blocking solar radiation from heating the surface, air pollutants lower surface heating and evaporation rates. This slows vertical air motions, and hence causes slower dispersal rates of air pollutants, and suppresses formation of convective clouds and precipitation. Reduced surface evaporation has major implications for the global hydrological cycle and how it responds to the combined forcing of GHGs, land use change, and aerosol pollution. In addition, surface deposition of dark aerosols accelerates ice-melt rates, hence affecting water resources. While these conclusions are based on sound physical meteorology, many of these effects are yet to be quantified.
4. The behavior of global averaged annual warming in the last several years has not conformed to their expectations. To attribute this absence of warming to the occurrence of La Niña, as some have done (e.g. see), is inappropriate as that climate feature is as much a part of climate system as any other component.
One positive aspect of their study, however, is we have a prediction of what the global annual average surface temperature anomaly should be in the coming years based on Figure 3 in the Huber and Knutti paper. Studies such as presented by Lucia at Blackboard, for example,
suggest this is going to be a challenge to verify. When the La Nina ends, as it inevitably will, we should see a rapid warming IF their analysis is correct. If the climate system does not rapidly warm, however, it will indicate we need to reject these studies in terms of their conclusion regarding the relative role of humans in global warming.