--> The Rock-Solid Scientific Evidence for Anthropogenic Global Warming

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The Rock-Solid Scientific Evidence for Anthropogenic Global Warming


Atmospheric CO2 concentrations have increased from 280 ppmv (parts per million by volume) prior to 1800 AD to almost 410 ppmv today, an increase of some 46%. Virtually all this increase has been caused by human activities. In the early 19th century, the main cause of the increase was deforestation, primarily in North America. Since then, the main driver has been consumption of fossil fuels. In the millennium prior to 1800, atmospheric CO2 oscillated between ~200 ppmv and 280 ppmv as part of the Pleistocene glacial-interglacial cycles. These data were obtained from polar ice cores. The last time CO2 levels exceeded 400 ppmv was probably during the mid-Pliocene, more than 3 m.y. ago, based on indirect proxy data. Global surface temperatures have also risen by about 0.8oC (1.5oF) since 1880. (The database is sparse before this time, as thermometers had just recently been invented.) About 2/3 of this increase has occurred since 1975. Some, or all, of the initial increase in surface temperature may have been a result of natural climate cycles, but the increase over the last four decades is almost certainly anthropogenic. Climate models that do not include the anthropogenic CO2 increase during this time interval do not reproduce the observed surface warming. Climate theory and observations are now in good agreement. What will happen to atmospheric CO2 and climate during the next century depends largely on how much fossil fuel we consume. The 2018 CO2 emission rate from fossil fuel burning was over 10 Gt(C)/yr, a new record. By comparison, volcanoes emit about 0.1 Gt(C)/yr, or 1% of the fossil fuel burning rate. The terrestrial biosphere emits about 60 Gt(C)/yr from respiration and decay, but this emission is almost completely balanced by CO2 uptake during photosynthesis. The preindustrial atmosphere contained about 600 Gt(C); the living biosphere (mostly trees) holds roughly this same amount. If we could substantially increase the amount of vegetation on Earth, we could postpone global warming temporarily; however, if we continue to deforest the globe, the global warming problem is simply exacerbated. The total economically recoverable fossil fuel reservoir is at least 5000 Gt(C). If we burn this all up over the next few centuries, atmospheric CO2 concentrations could rise to almost 2000 ppmv, just under 8 times the preindustrial level. Surface temperatures are predicted to increase by (31.5)oC per CO2 doubling, so 3 such doublings could produce a warming of ~9oC, or 16oF. This is not an acceptable outcome, as it would make much of the world uninhabitable by humans who do not have access to air conditioning. Reports written by the Intergovernmental Panel on Climate Change (IPCC) are a definitive source of information on past changes in CO2 and climate, and they represent consensus views about how these variables might change in the future. The IPCC reports typically make projections through the year 2100. The 2013 report presents several future CO2 emission scenarios. The most pessimistic (‘business as usual’) scenario, RCP8.5, envisions emissions climbing to 25 Gt(C)/yr by 2100, with atmospheric CO2 concentrations reaching 950 ppm. The associated increase in global mean surface temperature is (41)oC, or roughly 7oF. Predicted sea level rise is ~0.7 m; however, this estimate does not include possible contributions from melting of the West Antarctic ice sheet and could be too low by a factor of several (private communication, R. Alley). Of the many possible adverse effects of climate warming, sea level rise and increased human heat stress are likely the two most consequential (personal opinion). In summary, human activities are currently warming the climate, with potential effects that are serious on the 50-100 year time scale and devastating on longer time scales. The largest contributor to global warming, by far, is the burning of fossil fuels. How exactly to deal with this problem is a topic for continued debate. To make progress in this debate, however, we must first acknowledge that the problem itself is real.