Washington: A new study has shown that changes in global (ocean and land) precipitation may have been directly affected by human activities and cannot be explained by natural variability alone.
Emissions of heat-trapping and ozone-depleting gases affect the distribution of precipitation through two mechanisms. Increasing temperatures are expected to make wet regions wetter and dry regions drier (thermodynamic changes); and changes in atmospheric circulation patterns will push storm tracks and subtropical dry zones toward the poles.
Lawrence Livermore National Laboratory lead author Kate Marvel said that both these changes are occurring simultaneously in global precipitation and this behaviour cannot be explained by natural variability alone, asserting that the external influences like the increase in greenhouse gases are responsible for the changes.
The team compared climate model predications with the Global Precipitation Climatology Project`s global observations, which span from 1979-2012, and found that natural variability (such as El Ninos and La Ninas) does not account for the changes in global precipitation patterns. While natural fluctuations in climate can lead to either intensification or poleward shifts in precipitation, it is very rare for the two effects to occur together naturally.
Marvel and Celine Bonfils, the other LLNL author, identified a fingerprint pattern that characterizes the simultaneous response of precipitation location and intensity to external forcing.
Marvel said that most previous work has focused on either thermodynamic or dynamic changes in isolation but by looking at both, we were able to identify a pattern of precipitation change that fits with what is expected from human-caused climate change.
By focusing on the underlying mechanisms that drive changes in global precipitation and by restricting the analysis to the large scales where there is confidence in the models` ability to reproduce the current climate, "we have shown that the changes observed in the satellite era are externally forced and likely to be from man," Bonfils said.
The study has been published in the journal Proceedings of the National Academy of Sciences.