Global warming, land drying and terrestrial carbon sinks weakening

About 28% of anthropogenic carbon emissions are absorbed by terrestrial vegetation annually. This form of carbon sequestration fluctuates greatly1, compared to oceanic carbon absorption, and is controlled mainly by the annual variability of weather2-4. Observational data5 from a global monitoring network indicate that the sensitivity of terrestrial carbon sequestration to mean annual temperature (T) breaks down at a threshold value of 16oC, above which terrestrial CO2 fluxes are controlled by dryness rather than temperature. Here we show that warming climate has moved the 16oC T latitudinal belt poleward. Since 1948 the land surface area with T greater than 16oC that is now subject to dryness control rather than temperature as the regulator of terrestrial carbon sinks has increased by 6%. If the present trend continues this extension is expected to double to encompass another 6% by 2050. Most of the land area subjected to this warming is arid and semiarid with land cover ranging from open shrublands to croplands, grasslands and savannah, ecosystems that are highly vulnerable to drought and at high risk of land degradation. A data-driven analysis shows that in the areas that are now dryness-controlled, net carbon sequestration is ~27% lower than in areas in which both temperature and dryness (T <16oC) regulate plant productivity. Because a shift of land area to dryness control reduces its CO2 extraction from the atmosphere, the warming-induced extension of dryness-controlled areas may be triggering a positive feedback accelerating global warming. Continued increases in land area with T>16oC has implications not only for positive feedback on climate change, but also for ecosystem integrity and land cover, particularly for pastoral populations in marginal lands.

Acknowledgement: This research was supported by PSC-CUNY award (PSC-CUNY-ENHC-44-83).

References:

Yi, C., et al. Climate control of terrestrial carbon exchange across biomes and continents, Environmental Research Letters, 5, doi: 10.1088/1748-9326/5/3/034007, 2010.

Yi, C., et al., Climate extremes and grassland potential productivity, Environmental Research Letters, 7, 035703 (6pp) doi:10.1088/1748-9326/7/3/035703, 2012.