Mother Nature To The Rescue?

Abstract

Now we come to the key issue. Many discussions about climate change turn to the well- known fact that (very) large CO2 fluctuations have happened in the geological past. This is then taken to imply that “we shouldn’t worry: nature has seen this all before, and will somehow clean up our external carbon emissions.” The veracity of this sentiment can be tested by considering the main mechanisms available in nature for extracting carbon from the atmosphere-ocean system. These are weathering, reforestation, and carbon burial in soils and sediments. In the next section, we look at the potential of these processes. Thereafter, we consider the case for human intervention, and potential ways forward. A first mechanism by which nature has dealt with past high- CO2 episodes is chemical weathering of rocks. In warmer and more humid climates, chemical weathering rates are increased, and this extracts CO2 from the atmosphere. However, CO2 removal through weathering at natural rates is an extremely slow process, which operates over hundreds of thousands to millions of years. Given time, there is no doubt that natural weathering will be capable of eventually removing the excess CO2, but this process is so slow that it offers no solace for the future, unless we are prepared to wait many hundreds of thousands of years. There may be some future in artificially increasing the weathering processes to remove anthropogenic carbon, but this is in its infancy—we will revisit this in sections 6.2 and 6.3. A second mechanism for carbon extraction from the atmosphere-ocean system concerns expansion of the biosphere, most notably through reforestation. We have discussed this before in terms of expansion and contraction of the biosphere during ice- age cycles. In today’s case, carbon extraction through biosphere expansion requires first that the industrial age’s trend of net deforestation is reversed. Interestingly, this actually may have happened at around 2003. Between 2003 and 2014, net global vegeta¬tion increased by about 4 GtC (i.e., at an average rate of about 0.4 GtC per year), due to a lucky combination of increased rainfall on the savannahs of Australia, Africa, and South America, regrowth of forests on abandoned farmland in Russia and former Soviet republics, and massive tree- planting projects in China.