By Andy Turner
I wrote a year or so ago about the forecast for the (then) forthcoming summer monsoon season. India was worried about the possible effect of a predicted El Niño on the rainfall total; El Niño is normally associated with droughts in the monsoon. But in the end the El Niño of 2014 never really got off the ground. Instead, it kept its heat stored under the surface of the Pacific Ocean, and climate scientists are now predicting an even larger El Niño later in 2015, causing consequent worry for failing rains in India.
But the effect of variations in temperatures of the tropical Pacific on the monsoon is not the only thing to worry about – and the season ahead is not the only time frame of concern to state and regional governments and the population. In the UK we often, quite rightly, focus on the effects of rising greenhouse gas emissions on climate. For India, much evidence tends to suggest that increased emissions of carbon dioxide will lead to warming and slight but noticeable increases in monsoon rainfall for the seasons in the future. When real observations of monsoon rainfall are looked at since the 1950s, however, we find it has gone down. This presents something of a conundrum for monsoon climate scientists. Yet greenhouse gases such as carbon dioxide are not the only source of emissions from humanity.
Evidence suggests that aerosols – particulates from pollution suspended in the atmosphere – can have an effect on monsoon rainfall. This matters because to greater or lesser degrees, industry has continued to develop over large parts of the world over the 20th century. This has happened particularly in the northern hemisphere and strongly in India and China, fuelled by the need to strengthen the economy and meet the needs of the growing local population.
Some of the key aerosol pollutants from industrial emissions are sulphates. long with other emissions such as soot from cooking fires, we already know that polluting aerosols represent a big challenge for air quality and human health. But sulphate aerosols in particular have a mirror effect on radiation from the sun, reflecting some of it back up to space and stopping it reaching the surface. Each summer the monsoon develops following rapid warming of the land surface (and atmosphere above) around northern India and the Tibetan Plateau. After simplifying a lot of science, this comes down to what we call a land-sea contrast in temperature. So anything that reduces the warming and the land-sea contrast could weaken the monsoon. The suggestion is that polluting aerosol emissions may have been to blame.
Our new study, led by Dr Liang Guo, looked at models used in the recent IPCC Fifth Assessment Report. We analysed experiments that attempt to reproduce long-term trends in the climate of the 20th century by varying greenhouse gases, aerosol pollutants and the output from the sun in accordance with measurements. We found that if only greenhouse gases are considered, Indian monsoon rainfall tends to increase towards the end of the 20th century, quite contrary to observed rain gauge measurements. Instead, if we consider only aerosol pollutants, the rainfall goes down, rather more than observed. If we put both these factors together we get an answer quite similar to the measurements. In a further part of the work, we compared different models and showed that only those that include the effects of aerosol pollution on clouds could reproduce the downward rainfall trends. It also turns out that the aerosol pollution coming from the rest of the northern hemisphere, as well as the strong emissions from India, both contribute to the reduced rain.
So in the future we can’t just worry about carbon dioxide and its effect on warming. We know that industrial sulphate pollution is bad for health – but it is also bad for the monsoon.