Extended Data Figure 6: Global map of regions where climatic conditions and soil types resemble those of southern Saskatchewan, Canada.
From: Decrease in CO2 efflux from northern hardwater lakes with increasing atmospheric warming
Hardwater lake distribution is not well quantified; however, this map depicts the region in which subsoil composition favours hardwater lakes and where climatic conditions produce substantial winter ice cover. Soil data originate from the FAO–UNESCO Soil Map of the World; regions highlighted in black have subsoil concentrations of CaCO3 in excess of 10% (for example Cambisol, Xerosol, Yarsol, Kastanozem and Chernozem soils). These data were overlain with temperature data (10 arcminute resolution, averaged monthly during 1950–2000) obtained from the WorldClim Global Climate Data that were restricted to regions where the monthly average temperature was below 0 °C for December–February (June–August for the Southern Hemisphere) but where the temperature was above 0 °C in October (April in the Southern Hemisphere), to exclude high-latitude lakes with permanent ice cover. The highlighted area (15,200,000 km2) has pronounced winter and calcareous soils, spanning the prairie and steppe regions of North America, South America, Europe and Asia. If we assume this region to have a similar surface water distribution to that of southern Saskatchewan, the area occupied by permanent lakes should be between 740,678 km2 (at 1:50,000 scale) and 538,892 km2 (at 1:250,000). If these basins also experienced a decline in CO2 efflux of 100 g C m−2 per summer during the past 15 years (Fig. 1d), global hardwater lakes may have sequestered 74.1 Mt (53.9 Mt at the coarser resolution) more C per summer than they did during the mid-1990s, a change greater than 5% of global efflux from dilute boreal lakes3,4. This value should increase in the future as ice cover declines.
