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The Intergovernmental Panel on Climate Change has been thinking about what it will take to reduce the <br />impact of climate change, and the consequences of failure. It plotted out a number of scenarios, projecting <br />various approaches to carbon emissions. In two possible futures, dubbed RCP6 and RCP4.5, emissions <br />begin declining by 2040 or 2060 and drop below present levels by the optimistic date of 2100. [1] (See figure <br />on left below.) But CO2 content in the atmosphere will not stop dropping by the year 2100; it will take another <br />century or more for CO2 levels to drop. So, the stopping distance for climate change is at best 50 to 100 <br />years. <br />We have a few decades from now to come to grips with CO2 emissions and haul them back down to levels <br />of previous centuries. Even if we do, from that point, CO2 continues to increase for 50 years, increasing the <br />impact of climate change for that whole time. But beyond 2100, under this scenario, the trajectory of CO2 <br />and climate is more hopeful, and some species, some corals, some rain forests, may be beginning to <br />recover. We can make the 22nd century a century of climate recovery. But only by acting by the mid-21st <br />century. <br />25 • <br />0 c 20 <br />15 <br />c 10 <br />5 <br />0� <br />• <br />5. <br />2000 2025 2050 2075 <br />••••• RCP2.6 <br />RCP4.5 <br />RCP6 <br />-- RCP8.S <br />c <br />0 <br />600 <br />4 <br />w0; <br />2000 2025 20 <br />_ ! 4 <br />2075 2 C <br />Figure: Predictions of CO2 emissions (left) and CO2 levels (right) based on various future scenarios of global response to <br />climate change. The RCP 8.5 scenario (solid line) is currently the most likely as it represents no future controls on emissions. <br />This scenario would lead to an exponential increase in oceanic CO2 with serious impact on global life from about 2100 <br />onward. Only if CO2 emissions were to begin to decline by 2020 (for example in the RCP 2.6 scenario (dotted lines)), would <br />CO2 in the atmosphere begin to decline by 2100. Intermediate scenarios (RCP 4.5 and RCP 6.0) would see CO2 levels <br />increasing for the foreseeable future. Figure from S. R. Palumbi and A. R. Palumbi, Extreme Life of the Sea, Princeton U. <br />Press. 2014. <br />