Forecasting global warming

A discovery by Arizona State University engineers Peter Crozier and James Anderson could lead to more accurate forecasting of global warming.

After studying aerosols in the atmosphere, the researchers believe some measures used in atmospheric science are oversimplified and overlook important factors that relate to climatic warming and cooling.

Today, studies of what directly contributes to climate change have focused on carbon dioxide and other greenhouse gases.

But there are other components in the atmosphere that can contribute to warming – or cooling – including carbonaceous and sulphate particles from combustion of fossil fuels and biomass, salts from oceans and dust from deserts.

Brown carbons, a nanoscale atmospheric aerosol species from combustion processes, are the least understood of these aerosol components and are largely being ignored in broad-ranging climate computer models.

The researchers say the effect of brown carbon is complex because it both cools the Earth’s surface and warms the atmosphere and that a key to understanding their properties is to look closely at their light-scattering and light-absorbing properties.

Anderson said: ‘If we know the optical properties and distribution of all the aerosols over the entire atmosphere, then we can produce more accurate climate change models.’

Most of the techniques used to measure optical properties of aerosols involve shining a laser through columns of air.

The problem is that this approach only provides the average properties of all aerosol components, and at only a few wavelengths of light.

Eschewing that approach, Anderson and Crozier used a monochromated electron energy-loss spectroscopy to directly determine the optical properties of individual brown carbon nanoparticles over the entire visible light spectrum, as well as over the ultraviolet and infrared areas of the spectrum.

Crozier said: ‘We have used this approach to determine the complete optical properties of individual brown carbon nanoparticles sampled from above the Yellow Sea during a large international climate change experiment.’

It’s typical for climate modellers to approximate atmospheric carbon aerosols as either non-absorbing or strongly absorbing.

Crozier added: ‘Our measurements show this approximation is too simple.

‘We have shown that many of the carbons in our sample have optical properties that are different from those usually assumed in climate models.’

The work is part of the Aerosol Characterisation Experiment (ACE) programme, which involves hundreds of researchers from many countries.