There is a good review of all the model projections in the IPCC Summary for Policymakers (https://www.ipcc.ch/pdf/assessment-report/ar5/syr/AR5_SYR_FINAL_SPM.pdf). See especially pages 8-10. As others have already said, there is a range of projections for any given assumed concentration of greenhouse gas emissions reflecting differences in the models. Confidence in projections also diminishes the further out in time and the greater the departure from historic experience. There are model projections showing 4C as soon as 2075 under high emission scenarios. There is no one scenario that can best be described as the “status quo” past about 2030 as the rate of emission growth becomes increasingly dependent on decisions yet to be made.
The IPCC developed Representative Concentration Pathways, or RCPs, to provide a set of consistent atmospheric concentrations of GHGs to be used by all the major climate models thus facilitating comparison. As the authors of the RCPs explain, “They should not be considered forecasts or absolute bounds. RCPs are representative of plausible alternative scenarios for the future but are not predictions or forecasts of future outcomes. No RCP is intended as a “best guess,” most likely, or most plausible projection.” You can find a detailed explanation of the RCPs and how they were prepared online at http://www.aimes.ucar.edu/docs/IPCC.meetingreport.final.pdf. The highest RCP which models show reaches or exceeds 4C by 2100 is based on expectations that fossil fuel use continues to rise through the century. The IPCC does not evaluate how likely this is, only what the consequences may be should it come about.
CO2e emissions have been generally tracking above the line for the RCP 8.5 scenario, which is a reason for some to predict 4C by 2050. The zero baseline for that graph is the 1986-2005 average. We have a 50-50 chance of passing 4C by 2082, and about three chances out of four that we pass 4C by 2100.
If we drastically reduce emissions of greenhouse gases very rapidly, then we can avoid passing 4C this century. But the CO2 levels in the atmosphere already basically ensure passing 4C is subsequent centuries. If we want to avoid 4C in the long run, then we must develop technologies for negative CO2 emissions, i.e., was to suck CO2 from the atmosphere and reduce the CO2 concentration below current levels. For safety, we should reduce the CO2 concentration to around 315 to 320 ppm.
The IPCC Summary for Policymakers is a political document based on science, but it is not a scientific gold standard. If you want something like a scientific “gold standard,” then you must go to the working group reports themselves, not the summaries.
Moreover, IPCC reports are already essentially obsolescent when they are released. The picture is changing rapidly, and it is necessary to stay familiar with the latest peer-reviewed literature to be not too far behind the current picture.
Aggregating model projections produces probabilistic estimates and that the IPCC has quantitative definitions for it’s statements about likelihood — “likely” is defined as greater than 66 percent, “very likely” greater than 90 percent. Thus while the median projected warming is 3.9C for a GHG concentration of 800 ppm, the chance of exceeding 6C is 11 percent at only 700 ppm. There is a good discussion of this distributional result, the “fat tail” phenomenon, in a new book “Climate Shock” by economists Gernot Wagner and Martin Weitzman.
the Japan Met, GISS in the USA, and Hadley in the UK all agree that the global average temperature has increased over the last 17 or 18 years. As shown in the Cowtan and Way paper, those agencies underestimate recent temperature increases because they do not include arctic temperatures, where the increase has been the fastest.
There is no reason to doubt that the atmospheric concentration of CO2 is the main “thermostat” controlling the average global temperature. Multiple lines of evidence including the Earth’s climate history over the last 3.5 billion years establish that scientific fact.
“Both atmospheric CO2 and climate change are accelerating. Climate scientists say we have years, not decades, to stabilize CO2 and other greenhouse gases.”
Negative emissions are possible. One large area to focus on is agriculture, currently contributing around 14% of CO2 emissions. It is possible to grow food, fiber, and fuel in ways that increase carbon storage in the soil. Such a conversion could change agriculture from a CO2 source into a CO2 sink. Unfortunately, the appropriate growing techniques vary according to local conditions. In other words, there is not a one-size-fits-all approach, quite the opposite of the input-intensive standard agricultural approach. See the 2013 UNCTAD report for more on this point.
There are also ways to accelerate rock weathering, which takes CO2 from the atmosphere. Negative CO2 emissions can be achieved without inventing new technologies. We only need to apply what we already have on a massive scale.
But first we must stop burning fossil carbon. That is a sine qua non.
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