Today, something a bit different for the Climate Letter. We’ll start with a new report from Politico covering the latest presumptions of a vital need to remove carbon dioxide from the atmosphere. There is a European Commission that takes this need very seriously and is actively investigating possible methods, for reasons that are clearly stated in the report:
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Note that these reasons were derived from a report published by the Commission’s Joint Research Centre in July. That report makes reference to the conclusions of a team of scientists from around the world who were seeking to identify bottlenecks toward achieving the goals set in Paris, expressed as follows: “They found that even with very strong efforts by all countries, including early and substantial strengthening of the intended nationally determined contributions, residual carbon emissions will reach around 1000 gigatons of CO2 by the end of the century.” By my reckoning those residuals alone are the equivalent of about thirty more years of today’s level of fossil fuel emissions, which would be large enough to add another half degree to temperature increases if that carbon is not removed in a timely manner. That is on top of the carbon that must be removed right now, before the residuals even begin to appear, in order to achieve the 0.5 target. Thus the overall hurdle that is created for “negative emissions” to leap over is indeed very, very high, and we have yet to discover a viable methodology. Here is the link to the Commission’s research report: https://ec.europa.eu/jrc/en/news/higher-ambition-needed-meet-paris-climate-targets
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This information presents a question that has not been given an answer. Even without the trillion tons of residual emissions, how much CO2 would really need to be removed from the atmosphere in order to keep our planet within the 1.5C limit? I have been trying to figure that out, making a few broad assumptions and doing some simple calculations (with no peer review to back things up). My conclusion: It seems that we have already raised the CO2 level in the atmosphere more than enough to reach 1.5C within a couple of decades. That was settled in the year 2013 when we passed through the 396 ppm level at the Mauna Loa observatory. ( https://www.esrl.noaa.gov/gmd/ccgg/trends/graph.html ) Why 396? First, because that number is half way between 280 and 560 on what is for this purpose the commonly used logarithmic scale. It so happens that a 1.5 degree increase is precisely half of a 3.0 degree increase, and 3.0 happens be the number that a great many scientific models seem to converge upon when making estimates of climate sensitivity, or the amount of temperature increase expected to occur from any doubling of the CO2 level. This particular definition focuses on the kinds of changes that should be completed within decades, setting aside the potential for other kinds of effects that would likely require centuries to unfold.
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I have taken this way of thinking one step further, based on the general assumption that this shorter-term type of sensitivity should spread evenly across CO2 increases from any level and regardless of regularity or time of duration. This creates what might be called the “12% rule,” which simply states that any time you add 12% to the CO2 level you have established the foundation for an additional 0.5C in the global temperature. Using the ordinary rule of 72, 12% (actually a bit more) compounded six times will give you a double, and the accumulation of six such gains of 0.5C will neatly bring you up to the 3.0 mark that is regularly used as the principal sensitivity estimate.
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An advantage of dividing sensitivity up into six small-sized slices this way is that some of the early slices from our era can now be tested for historical accuracy. Beginning in 1750, when CO2 was still at 280 ppm, the first slice ended in 1958, at 314. A temperature increase of 0.5C, which cannot be measured very perfectly, appeared around 1990. The next 12% slice ended in 1989 at 352, with temperature adding another 0.5C just recently, in 2015. The third slice, which also completes the halfway point, ended at 396 in 2013 as observed above. With temperature rising steadily at a rate of 0.17C per decade we should reach 1.5C (from 1.1 today) right around 2040. Based on these results, I am not sure there is any way to improve on the 12% rule, so will leave it there for now, keeping in mind the possibility that bits and pieces of the much longer term “Earth System Sensitivity” impacts could start showing up at any time.
Carl
Carl