While the continents and oceans have absorbed much of the carbon dioxide that humanity has pumped into the atmosphere so far, they won’t be able to keep up with the expected rise in greenhouse-gas emissions over the next several decades.  Recent studies suggest that current estimates about natural absorption are too optimistic: Earth’s climate by century’s end could be on average up to 1.5 degrees C (2.7 degrees F.) hotter than current “business as usual” projections suggest.