tirsdag 15. mars 2016

Better to focus on the CO2 emissions than on the temperature

We tend to focus on the global surface temperature when we worry about man-made climate change. But it is better to focus on how much more carbon we can emit to the atmosphere before we are doomed to an unacceptable warming. This is because the warming is delayed with decades and centuries relative to the emissions, and because there is a natural variability in the temperature in addition to the long term warming trend. It is better to to concentrate on our work task, which is to reduce the emissions.

The ambitious goal in the Paris agreement (COP21) is to hold 'the increase in the global average temperature ... well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels' 1. There is a huge gap between this goal and the present actions and short term politics. A possible explanation of the gap may be that we wrongly believe that can wait some years, maybe even a decade, before drastic reductions in the emissions must be implemented. This misunderstanding may be fueled by focusing too much on the temperature, which increases rather slowly due to the inertia in the climate system.

Figure 1: The global surface temperature since 1880 shown as the average of three global surface temperature series. They are deviations (anomalies) from the mean temperature in the baseline period 1880 to 1909. The three series are from NASA, from the British Met Office and from NOAA.

Figure 1 shows the global surface temperature since pre-industrial time 2. The red line shows the long term temperature trend, and in January 2016 it is 1 °C warmer than the pre-industrial baseline. The monthly temperatures and the one year running average vary due to natural variability. Our climate concern is the long term warming trend, not the natural variations.

Despite more than one hundred years with man-made carbon emissions the temperature has increased by only 1 °C. In the next decade it will probably rise by another 0.2 °C, and there is still 0.3 °C left before the 1.5 °C limit is reached. We get a more grim and realistic view on the situation if we focus on the carbon emissions, as will be done in the rest of this post.

The man-made carbon emissions cause the atmospheric CO2 level to increase. Natural processes need one hundred thousand years to reduce the CO2 level back to what it would have been without the man-made emissions 3. Therefore, in the long term, it is the sum of the emissions that matters, not the emissions in specific years. The sum until a particular year is called the cumulative emissions in that year. A carbon budget is a sum of all emissions, including future emissions, that must not be exceeded if the warming shall be limited to a specific value. There is one budget for 1.5 °C and another budget for 2 °C.

Figure 2 shows the cumulative emissions from the burning of fossil fuel and production of cement. The data are from CDIAC 4, and they are updated till 2014, inclusive. The dotted blue line shows the cumulative emissions in the rest of the century if the emissions continue as they were in 2014 5.
Figure 2: The blue line shows the cumulative carbon emissions till 2014, inclusive. The unit GtC means billion tonnes of carbon. The dotted blue line shows future cumulative emissions in a scenario with yearly emissions as they were in 2014 (9.79 GtC / yr). The dotted black lines show carbon budgets for one and a half, two and three degrees warming; the horizontal lines are the budgets and the vertical lines show when the budgets are exhausted.

IPCC's last assessment report and an online climate model indicate that future emissions as shown with the dotted blue line in Figure 2 may result in a temperature increase as shown with the dotted blue line in Figure 1. They also indicate that the future emissions in Figure 2 may be likely in a scenario in which we try to reduce the emissions, but without adopting harsh measures to do so 6.

The dotted black lines in Figure 2 show the carbon budgets that with 66 % probability will limit the warming to one and a half, two and three degrees, as derived from the last IPCC assessment report 7. The budget for one and a half degree warming is exhausted in six years, and the budget for two degrees in a little more than twenty years.

When we focus on the emissions, as we do in Figure 2, we see that drastic reductions in the emissions have to be implemented immediately. Figure 1 does not tell this so clearly.


1 The 2 and 1.5 °C goals are described in article 2 in 'ADOPTION OF THE PARIS AGREEMENT' http://unfccc.int/resource/docs/2015/cop21/eng/l09r01.pdf

2 Figure 1 shows the average of three series with the global surface temperature. The temperatures are the anomalies relative to the reference period 1880 till 1909, inclusive. The series are GISTEMP from NASA, GlobalTemp from NOAA and HadCRUT4 from Met Office. http://data.giss.nasa.gov/gistemp/

3 Figure 10-4 in 'Global Warming: Understanding the Forecast 2nd Edition' by David Archer shows the longevity of an increased atmospheric CO2 concentration due to burning of fossil fuel.
Box 6 in IPCC's last assessment report provides the same information. It shows that the processes that reduce the CO2 concentration have timescales between on year and one million years.

4 CDIAC is an abbreviation for Carbon Dioxide Information Analysis Center, see http://cdiac.ornl.gov/. The data used to calculate the blue line in Figure 2 are fetched from the spreadsheet '2015 Global Budget V1.1', see http://cdiac.ornl.gov/GCP/carbonbudget/2015/

5 Figure 2 shows that the carbon emissions have risen exponentially. But the emissions have not increased so much in recent years as they did before, and the emissions in 2014 were only slightly higher than they were in 2013. A linear extension of the curve of the cumulative emissions may therefore be likely in a scenario in which we try to reduce the emissions, but without adopting harsh measures to do so. This is illustrated by the dotted blue line in Figure 2, with annual emissions ahead as they were in 2014.

6 The projection for the global temperature depends on the emissions.
- IPCC works with four emission scenarios from now till 2100. In the scenario with the second highest emissions, RCP6.0, the yearly emissions from the burning of fossil fuel and production of cement are a little greater than they were in 2014. In the same scenario the temperature projection is a little higher than the dotted line in my Figure 1. See Figure 6.25 and Figure 1 i FAQ 12.1 in
- ISAM is an online climate model at the home page of The University of Chicago.
http://climatemodels.uchicago.edu/isam/. The user can select one of four emission scenarios, and he can adjust the emissions within these scenarios. The second lowest, 'Low business as usual Scenario', resembles the dotted line in Figure 2. In that scenario the temperature projection is approximately as the dotted blue line in Figure 1. When the emissions in the 'Low Business as Usual scenario' are adjusted to 9.79 GtC per year, i.e. as the dotted line in Figure 2, the temperature projection is almost identical to the dotted blue line in Figure 1.

7 IPCC quantifies the carbon budgets in the 'Physical Science Basis' report and in the 'Synthesis Report' . The carbon budgets in Figure 2 are based on the last part of Table 2.2 in http://www.ipcc.ch/report/ar5/syr/ .The table tells how much carbon that can be emitted from 2011 if the warming with 66 % probability shall be limited to 1.5, 2 and 3 °C. (Strictly speaking, 66% is not a probability, but the proportion of the model simulations that keep the warming below the temperatures.) The values in the table are added to the cumulative emissions till 2011, inclusive, and the sums are plotted as the horizontal black dotted lines in Figure 2.

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