Warming climate extends dryness-controlled areas of terrestrial carbon sequestration

At biome-scale, terrestrial carbon uptake is controlled mainly by weather variability. Observational data 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 16°C, above which terrestrial CO2 fluxes are controlled by dryness rather than temperature. Here we show that since 1948 warming climate has moved the 16°C T latitudinal belt poleward. Land surface area with T > 16°C and now subject to dryness control rather than temperature as the regulator of carbon uptake has increased by 6% and is expected to increase by at least another 8% by 2050. Most of the land area subjected to this warming is arid or semiarid with ecosystems that are highly vulnerable to drought and land degradation. In areas now dryness-controlled, net carbon uptake is ~27% lower than in areas in which both temperature and dryness (T < 16°C) regulate plant productivity. This warming-induced extension of dryness-controlled areas may be triggering a positive feedback accelerating global warming. Continued increases in land area with T > 16°C 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.

1948-2010 with 0.5u 3 0.5u spatial resolution in the shifted area. Dryness was calculated from the monthly datasets of net incoming short-wave radiation and net long-wave outgoing radiation of the NCEP/NCAR reanalysis 8 and the gridded monthly terrestrial precipitation datasets 9 . This data-driven estimate indicates that CO 2 transfer from the atmosphere to the biosphere is reduced by 27% in the shifted area where T changed from less than to greater than16uC. Qualitatively, the model prediction reveals a positive feedback mechanism: climate warming extends the dryness-control area, which reduces CO 2 transfer from the atmosphere to biosphere. Thus, because the atmopheric CO 2 concentration will increase at a greater rate, the climate will warm at an accelerating rate due to the positive feedback. If the global area under drynesscontrol (Fig. 1a) continues to increase at only the same rate as and land average temperature (red line); and (b) correlation between annual dryness-control area and annual land-average temperature (R 2 5 0.90, P , 0.0001). The dryness-control area refers to the total area of regions where mean annual temperature was higher than or equal to 16uC and terrestrial CO 2 fluxes are controlled by dryness rather than temperature based on the direct observational evidence provided by a global monitoring network 5 . The annual dryness-control area and annual land surface temperature during the period from 1948 to 2012 were derived from mean monthly temperature data at surface (0.5u 3 0.5u resolution) from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP/ NCAR) reanalysis data set [6][7] . The black lines indicate the trends of dryness-control area that was similar to that of land-average temperature (omitted): a slight drop between 1948-1975 and then a striking increase during 1976-2012. The striking increase in temperature is a direct result of increased greenhouse gases in the atmosphere 31 . The red arrows in (a) indicate El Niño years with oceanic Niño index (ONI) greater than 11.0, while blue arrows in (a) indicate La Nina with ONI less than 21.0 (http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml). 70% El Niño years were consistent with warmer years, while 80% La Nina years were consistent with cooler years. occurred over the preceeding half-century, the warming-induced dryness-controlled area will double by 2050.
With climate warming much of Earth's land has been moderately drying since 1976, averged over all land areas, based on annual Palmer Drought Severity Index (PDSI) estimates (Fig. 2a) derived from the monthly self-calibrated PDSI data 0.5u 3 0.5u resolution over the spatial range from 60uS to 70uN [10][11] . Our analysis finds that the drying trend in the shifted area was strongest and in land areas where T is above 16uC was second strongest (Fig. 2b). The land area where T . 16uC encompasses low latitudes in the northern hemisphere, most of Africa, Middle and South America, Australia, Southand Southeast Asia (Fig. 3). In these regions, tower-based FLUXNET (a) Links between PDSI and ENSO events.The red curve shows PDSI for the area with temperature above 16uC; the green curve for the area with temperature below 16uC; the grey curve for the whole area of the land; and the blue curve for the shifted area from below 16uC to above 16uC during 1948-2012. The red arrows indicate El Niño years with oceanic Niño index (ONI) greater than 11.0, while blue arrows indicate La Niña with ONI less than 21.0 (http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml). (b) The trends of PDSI. The filled circles are five-year moving average of the PDSI data shown in (a). Mean annual land surface temperature during the period from 1948 to 2012 was derived from mean monthly temperature data at surface (0.5u 3 0.5u resolution) from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP/NCAR) reanalysis data set [6][7] . Annual PDSI data were derived from the monthly self-calibrated PDSI data (0.5u 3 0.5u resolution, spatial range from 60uS to 70uN, http://www.cgd.ucar.edu/cas/catalog/climind/pdsi.html) 11 10 . The PDSI behaviours to the ENSO events were different between: (1) the area above 16uC (red curve in (a)), 90% El Nino years were dryer, while 70% La Nina years were wetter; and (2) the area below 16uC (green curve (a)), 50% El Nino years were wetter, while 40% La Nina years were dryer (see Table 1).
www.nature.com/scientificreports SCIENTIFIC REPORTS | 4 : 5472 | DOI: 10.1038/srep05472 observations 12 document that at ground-level these large land areas indeed are drying up and this is confirmed by remote sensing data 13 . This drying is attributed to increased evaporation and evapotranspiration due to warming. If the trend of drying up over the large land area where T . 16uC continues a strong positive feedback on warming is suggested because of reduced CO 2 transfer from the atmosphere to land (expansion of the brown areas in Fig. 3) via NEE that is limited substantially by water availability (see Fig. 2b in Ref. 5), thus inducing additional warming. In contrast, in the land area where mean annual temperature is below 16uC (green area in Figure 3) a trend toward greater wetness has been observed with climate warming (Fig. 2).
Two large areas between the cold (,16uC, green color in Fig. 3) and warm zones (.16uC, brown color in Fig. 3) have different performances during El Niño/Southern Oscillation (ENSO) events (Fig. 2a, Table 1). In this analysis we included El Niño years with an oceanic Niño index (ONI) greater than 11.0 during the period between 1948 and 2012, and La Niña years with ONI less than 21.0 ( Table 1). Half of the El Niño years were consistent with the cold phases (dips of temperature curve) of the cold part of the land (CPL, green area in Fig. 3), while 70% of the La Niña years were consistent with the warm phases (peaks of temperature curve) of the CPL ( Table 1). The CPL warm/cold phases appeared to be opposite of the warm/cold phases of the ENSO cycle. However, the global land area followed the warm/cold phases of the ENSO cycle very well. The CPL wet/dry phases appeared different from that of the global land area (Table 1). However, the wet/dry phases of the warm part of the land (WPL) were very consistent with that of the ENSO cycle, i. e. 90% El Niño years were in the dry phases, while 70% La Niña years were in the WPL wet phases (Fig. 2a). We could not find a better relationship of the WPL warm/cold phases with the ENSO cycle. However, if we assume that WPL temperature responses to the ENSO cycle lag by a year, 90% of El Nino years coincided with the WPL warm phases, while 80% of La Niña years coincided with the WPL cold phases. These fascinating coincidences, that became obvious after lagging the data by a year, can be understood at least theoretically in the following way. In the WPL wet phases, a much larger fraction of net radiation is used for evapotranspiration as latent heat and hence potential warming is reduced, while in the WPL dry phases, comparitively less net radiation is used as latent heat, so the temperature is increased. This energy budget adjustment may need about a year to reach equilibrium for about half of Earth's land. Temperature responses to the ENSO cycle of the shifted area (purple color in Fig. 3) were similar to the responses of the total global land area because the temperature of the shifted area is close to the landaverage temperature. However, the pattern with 60% of El Niño years being wetter while 30% La Niña years were dryer for the shifted area is coincident with the typical precipitation patterns of the ENSO cycle reported by NOAA 14 .
The shifted areas are transitional zones where not only is the climate-control mechanism of NEE switched as discussed above, but also meteorological conditions are more variable and vegetation is highly vunerable to climate changes and weather extremes. Dominant vegetations in the shifted regions (purple color in Fig. 3) are open shrublands (25%), croplands (22%), grasslands (7%), and desert (13%) (see Supplementary Table 1). Except for the shifted areas in southeastern China (box 4 in Fig. 3) and southeastern United States (box 1 in Fig. 3), most shifted areas are arid and semi-arid land with typical vegetation of open shrublands and grasslands. The annual NEE of these ecosystems is quite sensitive to climate conditions of low precipitation and high evapotranspiration rates [15][16][17][18] .
The locations of shifted areas in the northern hemisphere are coincident with the descending branch of the Hadley cell (HC) and are consequently associated with low precipitation and high evaporation rates 19 . Several lines of evidence indicate that the HC has intensified and expanded poleward over the past three decades as a consequence of climate warming [20][21][22] and that the HC expansion in the northern hemisphere is stronger than in southern hemisphere 23 . The contemporaneous poleward shift of both the HC and the WPL (significant since late 1970s) and location of the WPL with respect to the HC (nothern desending branch of the HC), strongly suggests that the WPL migration poleward is driven by global warming. The drying trend of the shifted area ( Fig. 2a-b) should be expected to result in vegetation cover shift, with decreased biodiversity and desertification. A line of evidence from remote sensing imagery indicates that drying is accelerating the degradation of vulnerable shrublands in some semiarid Mediterranean area [24][25] .
Division of the land into the WPL and CPL by threshold value (16uC) of annual mean temperature based on 64 years (1948-2012) climate data brings new insights into the warming of Earth's surface. The two parts of the land behave almost opposite to each other in the phases of the ENSO cycle and differ in climate control mechanisms of carbon sequestration 5,26,27 . The WPL has expanded poleward significantly ( Fig. 1) and has become dryer ( Fig. 2) in the past four decades. The trend of warming-induced drying of the WPL, by reducing NEE thereby reducing withdraw of CO 2 from the atmosphere, contributes a positive feedback on global warming. Furthermore, as lands are shifted from CPL to WPL becoming more arid and subject to desertification, they also release soil carbon adding additional CO 2 to the atmosphre. It is estimated that 19-29 Pg of carbon were added to the atmosphere from vegetation and soil carbon pools globally by desertification 28 . The frontal boundary (or the shifted area) of the WPL has been transformed by global warming into more vunerable regions where weather gradients are stronger (Fig. 2), ecosystems are more sensitive to even slight increases in water deficit (Fig. 3) 25 , crop yield is reduced by extreme heat waves 29 , and vegitated land cover and pastroral population are reduced. For instance, in Australia, where wide areas are becoming not suitable for sheep breeding due to reduced precipitation and increased soil salinity. An expansion of the global network 30 monitoring NEE to target the identified shifted areas would provide data that could improve our ability both to model these regions as they undergo further transitions and to assess the likely impacts on climate as a consequence of altered NEE and increased soil aridity. The present work raises the following two questions: (1) what atmospheric circulation mechanisms support the hypothesis of a year time lag between the WPL temperature response and the ENSO water phases; and (2) is the synergistic poleward expansion of the frontal boundary of the WPL with the HC a long-term or a short-term behavior and what are the consequences of this synergy for global NEE and for the rate of change in atmospheric CO 2 ?