Temperature and rainfall extremes in a warming world

heavy rain

By Dr Flora MacTavish

Governments, planning authorities, companies and individuals all need information about  the impact of climate change on extreme weather in the future. A recent paper [1] investigates changes in extremes both globally and locally.

We can be relatively certain of the signature of climate change on a global or continental scale. On the other hand, estimating changes on a country-wide scale is harder and estimating them on a local scale (i.e. to the nearest few kilometres) is very difficult.

In the new study, scientists calculated the proportion of global land area in which certain weather extremes are expected to increase. This can be projected more accurately than the change at an individual location.

The researchers found that the hottest temperatures will get even hotter in half of the global land area within 30 years. This is more useful than simply saying that extreme temperatures will become hotter on average globally. It is also more accurate than making predictions about particular locations.

What impact is climate change having on extreme temperatures and precipitation?

Climate change is already having an impact on the weather we experience. As mean temperatures have increased, the extreme hottest and coldest temperatures have also gone up.

Rainfall patterns have shifted, and there has been an increase in heavy rainfall over most land areas since 1950 (IPCC, 2013).

Globally, these trends are expected to continue over the coming decades. Future trends at a local scale may be obscured by the natural variability of the climate.

Why is this paper interesting?

This study confirms that there are significant uncertainties in how extreme temperature and rainfall will change locally. In some regions, there will probably be no change in extreme weather over the next thirty to fifty years. A small proportion of places may even see a reduction in extreme temperatures or heavy rainfall  events over this time period.

Importantly, these uncertainties are mainly due to the natural variability of the climate. Uncertainties in climate models are much less important. Even a perfect model could not accurately project changes in extremes locally on a thirty to fifty year timescale.

Despite this uncertainty, the study found that it is possible to estimate changes in extreme weather as a proportion of global land area. This technique turned out to be more reliable than making projections for individual regions. It is also more informative than the global average change in extremes.

This technique is not completely new, but the paper adds to a body of research looking at how to predict future extremes.

What does this mean for decision makers?

Reinsurance firms and commodities traders both operate in global markets. They could use this technique to evaluate changes in risk, helping to set premiums or prices more appropriately.

Local planners will probably still opt to build resilience to climate threats. This study does not provide any new information about the impacts of climate change on a local level.

This study does however provide further evidence that it is not possible to predict the pace of change locally. There is little reason to wait for more certainty in the science before beginning to build in greater resilience to extremes.

What methods were used in the paper?

The paper measures the proportion of land that will experience an increase or decrease in extremes. The paper assesses changes in the frequency of:

  • Hot extremes: highest daily maximum temperature in a given year.
  • Cold extremes: coldest daily minimum temperature in a given year.
  • Intensity of heavy precipitation: annual maximum amount of precipitation falling within five consecutive days.
  • Dry spell length: annual maximum number of consecutive dry days, where a dry day has less than 1 mm of precipitation.

Climate models were used to estimate the proportion of global land area in which the above four measures will increase or decrease, and by how much. Firstly, the results of computational simulations using many different climate models were analysed to provide a measure of model uncertainty.

Secondly, the same climate model was run several times with slightly different atmospheric initial conditions. This provided a measure of the uncertainty due to climate variability.

What were the main conclusions?

There are significant uncertainties in how temperature and precipitation extremes will change on a regional scale. These uncertainties are largely due to climate variability, rather than any errors in how models represent climate processes.

The proportion of global land area that will be affected by changes in extremes can be estimated much more reliably. This technique should prove useful to organisations who can make use of global scale information about climate change impacts.

Half of land areas are expected to have hotter temperature extremes within 30 years.

By the period 2016-2035 there is expected to be an increase in the proportion of land areas that will experience intense precipitation.

Further reading from the Grantham Institute

Grantham Briefing Note 1: The slowdown in global mean surface temperature rise

Grantham Briefing Note 5: The changing water cycle

References

[1] Robust spatially aggregated projections of climate extremes, E. M. Fischer, U. Beyerle & R. Knutti.  Nature Climate Change (2013).

[2] IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T. F., D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P. M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, in press. Available on the web at: http://www.climatechange2013.org/report/review-drafts/.

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