A climate model based on the "global energy balance" has provided new evidence for human-induced climate change, according to its creators. Using this simple model, researchers in Switzerland conclude that it is extremely likely (>95% probability) that at least 74% of the observed warming since 1950 has been caused by human activity.

Previously, climate scientists have used a technique called "optimal fingerprinting" to pinpoint the causes of global warming. This involves using complex models to simulate the climate response to different "forcings". These include greenhouse gases, aerosols and ozone, as well as natural factors such as solar and volcanic variability. The relative contribution of each forcing is then assessed by a statistical comparison of the model outputs to the real-life warming pattern.

However, this method relies on the ability of climate models to accurately simulate the response patterns to each forcing, and also assumes that the responses can be scaled and added. Furthermore, changes in the energy balance of the climate system are not explicitly considered.

A conservative model

Now, Reto Knutti and Markus Huber at the Institute for Atmospheric and Climate Science in Zurich, Switzerland, have developed a model based on the simple fact that Earth's energy must be conserved. When the Earth is in equilibrium, the thermal energy it emits is equal to the amount of energy received from the Sun. However, evidence shows that this energy balance has become disrupted, with less energy being emitted back into space. The trapped energy in the climate system thus acts to heat up our planet, causing a rise in global temperature.

The researchers used their energy-balance model to investigate the cause and magnitude of this warming. The model, driven by observational records of climate forcings, surface temperature and ocean heat uptake, was run many thousands of times with different parameter combinations. The combinations that best matched the observations were then fed through the model a second time in order to simulate the climate response to each individual forcing.

The model predicts a global temperature increase of 0.51 °C since the 1950s, similar to the observed estimate of 0.55 °C. Greenhouse gases provide the largest contribution to this warming, responsible for a temperature increase of 0.85 °C, with approximately half of this greenhouse warming offset by the negative forcing of aerosols. On the other hand, the contribution of solar and volcanic forcing was close to zero.

Different but similar

The model was also used to simulate the future evolution of the climate system. A temperature increase of 1.29 °C was found for 2050–2059 compared with the 2000s, almost entirely due to greenhouse gases, with carbon dioxide being the dominant contributor.

These findings are consistent with the latest report by the Intergovernmental Panel on Climate Change (IPCC), as well as other studies that use the optimal fingerprinting approach. The researchers believe that their energy-balance model can be used in tandem with alternative climate-attribution methods.

"We don't criticize optimal fingerprinting – it is a very powerful technique – but to almost all people it's a black box," says Reto Knutti. "It's statistically complex, makes a number of assumptions and is not physically intuitive. At least to a physicist, conservation of energy is fundamental. The fact that our results are entirely consistent with optimal fingerprinting is an argument for even higher confidence in human-induced climate change."

"Independent evidence"

Paul Williams, a Royal Society University Research Fellow in climate modelling at the University of Reading, UK, agrees that the model is a useful tool. "Even the most hardened climate sceptic with a basic knowledge of physics could not possibly object to the application of energy conservation to the climate problem," he says. "The energy-balance method provides further independent evidence for the anthropogenic origin of the majority of 20th-century global climate change."

This study comes less than two months after the Berkeley Earth Surface Temperature project announced its preliminary findings. Motivated by criticisms of the current temperature datasets, this independent study is building a historical temperature record from scratch, using measurements from more than 39,000 stations. Its first results, for land-surface temperatures only, reveal a global warming of 0.91 °C over the past 50 years, in close agreement with previous observational records. And now, the study by Knutti and Huber provides new evidence that this warming is due to human activity.

The research is described in Nature Geoscience 10.1038/NGEO1327.