Unravelling a mystery of the global carbon cycle

Dr Chris Huntingford, a climate modeller at the Centre for Ecology & Hydrology, is co-author of a new Nature paper published this week. He explains more about how the research offers new clues into how the land carbon sink is regulated:

Flux tower in the Amazon rainforest
“FLUXNET” towers capture detailed but localised measurements of land-atmosphere exchanges of carbon dioxide. There is now a large network of these operating across the planet. These allow investigation of how year-to-year variations in surface meteorology affect ecosystems and their role in the global carbon cycle.

A paper has just been published in Nature entitled, “Compensatory water effects link yearly global land CO2 sink changes to temperature”, led by the Max Planck Institute in Germany and on which CEH has co-authorship. This uses simultaneous detailed measurements of land-atmosphere CO2 fluxes along with known weather conditions, and when merged with models of how land ecosystems function, gives an overall detailed map of annual plant productivity at different places. Places with higher levels of productivity are such that more CO2 has been “drawn down” from the atmosphere.

Extraction from the atmosphere of carbon dioxide means that emissions from fossil fuel burning are being at least partially offset, and a key question asked of climate research is the extent to which that will continue in the future. Analysis of present-day fluctuations of plant productivity in response to weather variation has the potential to give important clues as to the future. At its most simplest, a hot year might provide information on how ecosystems function, should they experience sustained ongoing high temperature years under global warming.

Until now, a mystery has surrounded this area of research. If looking at local sites – for instance, studying measurements from single towers of local land-atmosphere CO2 fluxes – then the dominant influence is how much it has rained in any particular year. Yet, at the global scale, strong correlations exist between the change in atmospheric CO2 concentration and whether the planet experienced a generally hot or cold year. This paper resolves that by analysing the amount of variation in vegetation productivity due to either rainfall or temperature fluctuations, and by considering a range of different length scales.

This new analysis shows a very clear transition can be observed, and whereby for larger continental scales and beyond, geographical rainfall variations tend to cancel. This then causes temperature levels to be the main determinant of productivity levels. This is a remarkable story, and as often happens, detailed analysis of components of the global carbon cycle can sometimes present new questions to be asked of meteorology. What is it about the atmospheric water cycle that suggests that at larger scales, higher rainfall over some land regions will often be cancelled by lower levels in adjacent areas?

Can analysis of the equations of atmospheric flow, water vapour and rainfall generation show conservation properties that explain the characteristic length scale found in this paper? Why does that particular scale and above cause the land to be more affected by heat than by rainfall?

Chris Huntingford

Additional information

Full paper reference: Jung, M et al. (2017). Compensatory water effects link yearly global land CO2 sink changes to temperature. Nature. DOI: 10.1038/nature20780

The Max Planck Institute for Biogeochemistry issued a press release for this story.

Staff page of Dr Chris Huntingford, CEH

Issues: