Swansea scientists discover more smoke means less rainfall
Scientists at Swansea University’s School of the Environment and Society have revealed a feedback link between lack of rainfall and the amount of biomass burning in the Amazon.
Dr Suzanne Bevan and Dr Peter North used a new satellite record over a 13-year period to discover a connection between smoke produced through burning the Amazon forest for farming purposes and low rainfall patterns.
Dr Bevan, who now uses satellite-borne radar to measure how fast glacier ice in the Arctic flows, said: “Just as in the Arctic, the Amazon is an area so vast and inaccessible that remote sensing from space is the best way to monitor it.”
“Our research concentrated on aerosols, tiny solid or liquid particles that float suspended in the atmosphere. Aerosol Optical Depth (AOD) is a measure of the total amount of sunlight scattered or absorbed by aerosols throughout the depth of the atmosphere allowing us to estimate how many aerosol particles are in the atmosphere at a particular moment.
“Aerosols also act as a cloud condensation ‘seed’ onto which water vapour can condense to form cloud droplets. This means adding aerosols to the atmosphere can change the properties of clouds, changing their reflectivity, their lifetimes and also their ability to produce precipitation.”
Dr Bevan and Dr North focused specifically on smoke aerosols which pollute the atmosphere during the dry season in the Amazon (August - October) when biomass (wood, foliage and other plant matter) is burnt in the process of deforestation.
The key question for the researchers was whether forest-burning during the dry season actually extends drought conditions through the creation of smoke aerosol particles.
This is difficult from space because satellites looking down from above the top of the atmosphere monitor light scattered back by both the Earth’s surface and the atmosphere at the same time.
“To solve this issue, we used an algorithmo use data from radiometer instruments designed in the UK, which have been flying onboard European Space Agency satellites since 1995.
“These instruments view the Earth from two different angles and use four different wavelengths (colours), letting us separate light scattered by the Earth's surface from light scattered by atmospheric aerosols. The rainfall observations we used are based on a combination of data from rain gauges and observations from satellite microwave and infrared instruments.
“Using this data, we discovered that from 1995 to the 2008 dry-season AOD was strongly correlated with the number of fires and the depleted levels of rainfall.
“The aerosols are able to suppress rainfall by reducing the effective size of cloud droplets and lead to a feedback effect between rainfall, fire and smoke production during the Amazon dry season.
“However, this link did not prove that the fires were preventing rainfall; the fires may simply have been spreading further due to the dry conditions.
“With our unique 13-year time series of dry-season AODs we were able to identify trends in biomass burning over several years, and to relate these to external factors.
“We found that when land values were increasing, due to worldwide demand for soybeans and beef, concentrations of aerosols in the atmosphere rose. More recently, from 2004 onwards, with the exception of 2005 when aerosol levels were very high probably due to a severe drought, falling soybean prices, a strong Brazilian currency and active government intervention reduced the demand for land and AODs declined as less forest was burned.
“The research demonstrates that feedback effects between drought and smoke production result in longer, more intense dry seasons when the number of fires and the concentrations of smoke aerosols are high. In this way biomass burning can amplify via climate feedback processes, the destruction of the rainforest caused by agricultural deforestation.
“Now that global demand is rising again for soybeans and beef, and for ethanol to replace fossil fuels, there is a risk that deforestation and burning will begin to rise again and exacerbate the warming and drying effects of global climate change on the Amazon rainforest, and hence lead to an increase in forest die back.
Dr Bevan and Dr North’s research is published in the November edition of PlanetEarth: http://planetearth.nerc.ac.uk/features/story.aspx?id=583
For further information about research at the School of Environment and Society, visit: http://www.swan.ac.uk/environment_society/