April 6, 2016

expert reaction to Northern Hemisphere hydroclimate variability

In order to make accurate simulations of our future global climate, experts have required suitable instruments to measure data from the past. In this study published in the journal Science researchers describe the degree of hydroclimate variability in the Northern Hemisphere over the past 1200 years.

Prof. Richard Allan, Professor of Climate Science at the University of Reading, said:

“Basic physics backed up by detailed simulations and observations point to an intensification of wet extremes in a warming world and evidence is also building that tropical dry seasons may also become more severe. The work by Ljungqvist and colleagues is important because it gathers valuable indirect ‘proxy’ measurements of climate spanning the last 1200 years and uses this to test the realism of detailed simulations that are used to make projections of future climate change.

“Although the authors do not find the strong increase in wet and dry extremes simulated for the 20th century in the proxy observations there are a number of possible reasons for the discrepancy. An apples to oranges comparison of indirect and local measures of hydroclimate with simulated rainfall is problematic and also susceptible to fluctuations in global weather patterns which tend to muddy human-caused climate change signals.”

Prof. Richard Harding, from the Centre for Ecology & Hydrology, said:

“This study presents an impressive collation and analysis of proxy climate records over the last millennia to provide a time series of wet and dry areas across the northern hemisphere. A comparison with six climate models shows a remarkable agreement through the early and middle part of the record.

“There is some divergence through the 19th and 20th century which illustrates again how far we still have to go to realistically simulate the hydrological cycle, particularly at the regional scale. The long-term observational record of main components of the hydrological cycle (rainfall, evaporation, river runoff and soil moisture) is woefully incomplete, particularly in sensitive semi-arid regions, and this study is a significant contribution to support this record.

“Despite its considerable scope the study relies on only 196 sites to cover the whole of the hemisphere – it is to be hoped that this study will encourage more paleo climate studies the fill many current gaps.”

Prof. Ted Shepherd, Grantham Professor of Climate Science at the University of Reading, said:

“This paper adds to the growing evidence that the simple paradigm of ‘wet-gets-wetter, dry-gets-drier’ under a warming climate does not apply over land areas, which are of course the areas of importance for climate change impacts. It also underscores the importance of very long observational data sets, such as those obtained from palaeoclimate records, for constraining aspects of climate such as droughts that undergo long-term natural variability.

“Although scientific understanding of climate change is sufficient to demonstrate the urgency of moving towards a carbon-neutral economy, adaption planning at national or regional level requires a more detailed understanding of the impacts of climate change, especially in the area of hydroclimate.”

‘Northern Hemisphere hydroclimate variability over the past twelve centuries’ by Fredrik Ljungqvist et al. published in Nature on Wednesday 6 April.  

Declared interests

None declared