Four scientists based at the Climate Research Division in Ontario, Canada have recently made this bold statement. They have carried out research looking at past changes in the characteristics of extreme precipitation events. For a number of years now it has been suggested that humans may have played a role through climate change in creating the increase in heavy precipitation, but as mentioned in previous posts it has always been difficult to actually prove.
In this study they used historical climate data from between 1951 and 1999, mainly focussing on the Northern Hemisphere where data is more readily available and accurate. They used observed daily observation data from over 6000 stations alongside the CMIP3 (Coupled Model Intercomparison Project) archive for model simulations.
In order to assess anthropogenic impact segments of the historical data were used in the simulations with either:
- Historical anthropogenic forcing (greenhouse gases, and other anthropogenic factors such as aerosols)
- Combination of historical natural (solar and volcanic forcing) plus anthropogenic forcing
- Unforced control simulations
They looked for ‘fingerprint patterns’ in the simulation outputs, and regression coefficients that were obtained were significantly greater than zero, indicating that anthropogenic influence is readily detectable from the historical data.
The above image displays geographical distribution of trends of extreme precipitation indices during 1951–99. A and b are Observations; C and D are model simulations with anthropogenic forcing; E and F are model simulations with anthropogenic plus natural forcing. For each pair of panels, results are shown for annual maximum daily (RX1D) and five-day (RX5D) precipitation amounts. The units used are per cent probability per year.
The paper outlines that these changes in extreme precipitation follow the Clausius-Clapeyron relationship. Whereby there is an exponential increase of atmospheric moisture with warming, as the atmosphere can hold more water at warmer temperatures.
What’s their punch line?
“Human-induced increases in greenhouse gases have contributed to the observed intensification of heavy precipitation events found over approximately two-thirds of data-covered parts of Northern Hemisphere land periods.”
Moreover, they claim that there results may in fact be an underestimate, due to the modelling process. As a result, extreme precipitation events may strengthen more quickly than what has been projected and may have more severe impacts than estimated. Either way it would appear that we are likely to experience an increase in extreme precipitation events over the coming decades if anthropogenic impacts continue.
As a final comment of caution, it is always worth remembering that there are always uncertainties and limitations in the observed data use in studies such as these. There are also questions over missing or uncertain external forcing’s which could account for some of the difference between the observed and simulated data. Models are also never perfect and its reliability also needs to be considered. Despite these things, this paper presents a significant step in what would appear to be the right direction and should be taken as scientific fact, unless proven otherwise.
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