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Possible effects of global warming (Rev #1, changes)

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This page will list the major possible effects of global warming and provide references to current research on this subject.

Extreme weather events

The role of global warming in causing floods, droughts and other extreme weather events is much argued. It is particularly difficult to attribute individual events to global warming, but in February 2011 two papers appeared on this topic.

  • Pardeep Pall, Tolu Aina, Dáithí Stone, Peter Stott, Toru Nozawa, Arno Hilberts, Dag Lohmann, and Myles Allen, Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000, Nature 470 (17 February 2011), 382–385.

Abstract: Interest in attributing the risk of damaging weather-related events to anthropogenic climate change is increasing. Yet climate models used to study the attribution problem typically do not resolve the weather systems associated with damaging events such as the UK floods of October and November 2000. Occurring during the wettest autumn in England and Wales since records began in 1766, these floods damaged nearly 10,000 properties across that region, disrupted services severely, and caused insured losses estimated at £1.3 billion. [] Here we present a multi-step, physically based ‘probabilistic event attribution’ framework showing that it is very likely that global anthropogenic greenhouse gas emissions substantially increased the risk of flood occurrence in England and Wales in autumn 2000.

This paper is not freely available online yet, but the supplementary online information is free and interesting. It begins:

A popular simple thermodynamic argument assumes precipitation extremes are constrained to change with the water vapour capacity of the atmosphere that can be determined, under conditions of constant relative humidity, using change in mean surface temperature alone according to the Clausius-Clapeyron relation38. This argument is typically invoked in the aftermath of floods as an explanation for possible increases in such severe wet events under an anthropogenically warming climate.

While this is an oversimplified treatment not fully accounting for the complex hydrometeorology typically associated with UK flooding, it may nevertheless provide a physically plausible first guess of increases in mid-latitude precipitation extremes under warming. Indeed, a recently updated analysis of observed atmospheric column water vapour for past decades finds increasing trends over the UK and western Europe, and a significant autumnal increase more generally over Europe and the Northern Hemisphere; and this appears in agreement with a similar analysis finding increases in observed atmospheric humidity under warming for these regions that are within expected moistening rates for near-constant relative humidity.This latter analysis in particular appears broadly consistent with observations of Clausius-Clapeyron scale increases in surface specific humidity (the principle source for the free-troposphere) under warming over past decades, again with near-constant relative humidity – including for an European region incorporating the UK. Since these surface specific humidity increases have been attributed to mainly anthropogenic drivers, this lends support to a thermodynamic mechanism for increasing UK precipitation, and hence flooding, under anthropogenic warming.

Here we use this thermodynamic argument to deduce the reduction in observed England and Wales total daily precipitation extremes for an autumn 2000 climate, had estimated twentieth-century surface warming attributable to anthropogenic greenhouse gas emissions not occurred. Then regarding this reduction in precipitation extremes as a direct measure of reduction in flooding, we calculate the fraction of attributable risk (FAR) of these extremes, and compare it to the FAR of autumn 2000 flooding explicitly modelled in terms of severe daily river runoff using our more rigorous multi-step probabilistic event attribution (PEA) framework of the main text.

This related abstract is also available:

The second paper appearing in February 2011 is:

  • Seung-Ki Min, Xuebin Zhang, Francis W. Zwiers and Gabriele C. Hegerl, Human contribution to more-intense precipitation extremes, Nature 470 (17 February 2011), 378-381.

Abstract: Here we show that 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 areas. […] Changes in extreme precipitation projected by models, and thus the impacts of future changes in extreme precipitation, may be underestimated because models seem to underestimate the observed increase in heavy precipitation with warming.

Again this paper is not yet available for free online, but there’s a discussion of it here:

category: climate