The role of anthropogenic climate change in the 2013 North Island drought

By Luke Harrington

A report released September 29^th 2014 by the Bulletin of the American Meteorological Society has addressed the causes of sixteen individual extreme weather events which occurred around the world in 2013, and specifically examined the role of anthropogenic (human-induced) climate change in each case. The report, “Explaining Extreme Events of 2013 from a Climate Perspective”, was compiled by 92 scientists worldwide and found a mixture of results when detecting a ‘climate change signal’ in an extreme event. The Guardian provides a good summary of the results here.

I was the lead author on an article within the report which focused on the North Island drought from the summer of 2013. The New Zealand Treasury estimates the drought cost the economy at least NZ$1.5 billion, with associated impacts expected for at least two years following the event. The role of our analysis was to understand how the likelihood of this type of event has changed as a direct result of anthropogenic changes to the climate system – this includes both greenhouse gas emissions and, because of our Southern Hemisphere location, ozone depletion.

Observational records found the highest number of dry days over the North Island in any given 3-month period on record, to have occurred during the months of January to March 2013.

To evaluate changes in the likelihood of this event from human-induced changes to the global climate system, we chose to look at two indicators of drought – a measure of surface pressure over the North Island, and the maximum 3-month accumulation of dry days in any given summer.

Using 17 different state-of-the-art climate models, we compared two types of simulations: those which included all natural and anthropogenic forcing mechanisms (including greenhouse gas emissions – this is our best estimate of the real world) and those which included natural forcings only – this is our best estimate of a world ‘that might have been’ in the absence of anthropogenic climate change.

Our results found a statistically significant (95% confidence) increase in the number of months with extreme surface pressure and dry days over the North Island, when comparing those model simulations which included anthropogenic climate forcings to those which did not. Furthermore, we found a very strong correlation between the increase in surface pressure and a corresponding increase in the number of days without rain over the North Island. This demonstrates that, in climate models at least, increased dryness with anthropogenic climate change is the result of more persistent subsidence over the North Island (essentially, this means longer-lasting and more intense high pressure systems).

It is important to realise that natural variability played a strong role in the severity of the 2013 drought – an event of this severity could have indeed occurred in a hypothetical world without human-induced climate warming. However, our results demonstrate that the North Island has experienced a systematic and statistically significant shift in several climate indices.  The meteorological conditions conducive to drought over the North Island have thus become more likely to occur, as a result of anthropogenic changes to the climate system.