Studies of radiative forcing components: Reducing uncertainty about climate change

Gunnar Myhre has been working to reduce the level of uncertainty in projections from the Intergovernmental Panel on Climate Change (IPCC). With funding from the Research Council of Norway's NORKLIMA Programme, he and his research colleagues have studied as many radiative forcing components as possible simultaneously.

Energy balance of the Earth system

Earth's temperature is determined by the difference between incoming solar energy and outgoing thermal radiation from Earth's surface and atmosphere that escapes into space. In its assessment reports, the IPCC estimates the energy balance using various radiative forcing components on Earth's climate.

Unsure of cooling effect

The IPCC's Fourth Assessment Report (2007) estimates the overall radiative forcing from anthropogenic carbon dioxide in the atmosphere to be 1.66 W/m². The combined forcing from other greenhouse gases (methane, nitrous oxide and halocarbons) was estimated at 0.98 W/m². The IPCC assigned medium confidence to these estimates.

The IPCC was far less certain about the cooling effect of various anthropogenic radiative forcing components, particularly atmospheric aerosols (tiny, floating particulates and droplets). In 2007 the IPCC estimated that anthropogenic aerosols have a direct and indirect cooling effect, with a radiative forcing of -1.2 W/m².

Global team effort

In recent years, scientists all around the world have taken part in a collective research effort to enhance knowledge about our planet's climate and to reduce uncertainty about the effects of various drivers of climate change. In the project Radiative forcing of climate change, Dr Gunnar Myhre and his colleagues have carried out climate modelling on a large scale with the aim of reducing the uncertainty associated with the degree of radiative forcing exerted by aerosols and ozone.

"Our objective has been to provide the IPCC with knowledge that enables them to produce better projections of climate change," says Dr Myhre.

While greenhouse gases warm our climate, and aerosols in all probability act to cool it, factors such as ozone and changes in albedo (the reflectivity of different surfaces) can work both ways.

Impact of aerosols

For many years, human activities have sent vast amounts of aerosols into the atmosphere in the form of sulphate, black carbon and organic particulates. These anthropogenic aerosols are emitted in addition to natural aerosols such as sea salt, sulphate particles in volcanic ash, and sand from the Sahara.

The Western countries have managed to lower their sulphate emissions dramatically. But China in particular is still emitting large amounts of both black carbon and sulphate, a situation causing tremendous health problems for millions of Chinese. So it is likely that within a few years, China will also focus on major reductions in emissions.

"When better sulphate abatement technology is available and emissions drop, it may lead to warming of the atmosphere," says Dr Myhre.

Can cool or warm the climate

Black carbon and sulphate are short-lived climate forcers, with atmospheric lifetimes of as little as a few weeks.

"More knowledge is needed about what will happen when atmospheric levels of black carbon and sulphate are reduced. They can either cool or warm the atmosphere -- but in all likelihood their primary effect is cooling," says Dr Myhre. In a 2009 article in the journal Science, he concluded that the amount of black carbon in the atmosphere may have increased sixfold since the industrial age began.

Less uncertainty now

Researchers in the project Radiative forcing of climate change have examined as many climate forcers as possible at the same time, and all of them were treated consistently.

"This makes it easier to understand the individual components, as well as how they affect each other and what uncertainties remain," explains Dr Myhre.

Dr Myhre has worked particularly on aerosols and ozone. He has collaborated with other modelling groups and has been involved in developing a new model for the direct effect of anthropogenic aerosols -- a model that is now proving very useful in the IPCC's work.

"Based on our findings from the radiative forcing project, we have been able to reduce the uncertainty associated with the direct effects of aerosols considerably. Their cooling effect has proved to be less than previously thought. Biomass burning may have a significant effect on the radiation budget, but the net result of the warming and cooling effects is close to zero."

Impacts still clouded by doubt

Since the publication of the IPCC's Fourth Assessment Report, climate scientists have learned more about the various anthropogenic climate forcers, but still much remains to be done, according to Gunnar Myhre.