Posted on 30 January 2017 by dana1981

The latest Intergovernmental Panel on Climate Change (IPCC) report – which summarizes the latest and greatest climate science research – was quite clear that humans are responsible for global warming:

It is extremely likely [95 percent confidence] more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together … The best estimate of the human-induced contribution to warming is similar to the observed warming over this period … The contribution from natural forcings is likely to be in the range of −0.1°C to 0.1°C, and from internal variability is likely to be in the range of −0.1°C to 0.1°C.

In fact, the report’s best estimate was that humans are responsible for all of the global warming since 1951, and greenhouse gases for about 140%. That’s because natural factors have had roughly zero net effect on temperatures during that time, and other human pollutants have had a significant cooling effect.

In other words, the Earth’s surface warmed about 0.65°C between 1951 and 2010. Human greenhouse gas emissions caused temperatures to be about 0.9°C hotter than they would have otherwise been. But other human pollutants caused about 0.25°C cooling, and natural factors had a very small effect.

Contributions to the 1951–2010 global surface warming. Illustration: IPCC Fifth Assessment Report

This expert conclusion is quite different from the comments of Trump’s cabinet nominees, who were all very consistent in admitting that there is “some connectivity,” as Trump put it, between human activity and global warming, but claimed that the degree of human influence remains open for debate.

Here’s the evidence

The IPCC conclusion on human-caused global warming rests upon a broad and deep base of evidence. To start, there’s basic math and physics. Long-term global temperature changes are a response to changes in the Earth’s energy balance. If there’s more incoming than outgoing energy, temperatures rise. For example, that happens when the sun becomes more active (more incoming energy), or when the greenhouse effect increases (trapping heat, decreasing outgoing energy).

Over the past 50 years, solar activity has been flat on average. Greenhouse gas levels in the atmosphere have skyrocketed due to humans burning fossil fuels. When scientists look at the various factors that contribute to the global energy balance, over the past 50 years, the increased greenhouse effect dwarfs all others.

The increase in heat trapped below the atmosphere and decrease in heat escaping to space are also two of the many “fingerprints” of human-caused global warming. These are changes scientists expect to see if human carbon pollution is the culprit behind global warming. 

As another example, we expect to see nights warming faster than days, because at night there’s a smaller volume of air, which allows the increased greenhouse effect to have a bigger effect on temperatures. Warming due to the sun would likely have the opposite effect, causing faster daytime temperature rise when sunlight is bombarding the Earth. For all of these “fingerprints,” observations are consistent with human-caused global warming and many are simply inconsistent with natural warming.

There have also been numerous studies specifically investigating and quantifying the various contributors to global warming. I summarized ten of these in the graphic below, and provided details about each study here. The results have been consistent with the IPCC conclusion – humans are responsible for essentially all of the global warming since 1950.

The percentage contribution to global warming over the past 50-65 years is shown in two categories: human causes (left) and natural causes (right), from various peer-reviewed studies (colors). The studies are Tett et al. 2000 (T00, dark blue), Meehl et al. 2004 (M04, red), Stone et al. 2007 (S07, green), Lean and Rind 2008 (LR08, purple), Huber and Knutti 2011 (HK11, light blue), Gillett et al. 2012 (G12, orange), Wigley and Santer 2012 (WG12, dark green), Jones et al. 2013 (J13, pink), IPCC AR5 (IPCC, light green), and Ribes et al. 2016 (R16, light purple). Illustration: Dana Nuccitelli

Scientific consensus arises from consilience of evidence

There is of course a 97% expert consensus on human-caused global warming. That consensus isn’t a result of groupthink, or a vast conspiracy, or money-grubbing scientists lying to get their greedy little hands on grant funds. In fact, scientists are very hard to convince of anything, and they’d much rather be the one to disprove a theory than the twenty-thousandth to reaffirm it. 

But scientists base their conclusions on evidence, and as discussed above, the evidence for human-caused global warming is overwhelming. In the video below from the Denial101x free online course, Peter Jacobs discusses the three pillars of this type of knowledge-based consensus.

Denial 101x knowledge-based consensus lecture by Peter Jacobs.

Verdict: Trump and his cabinet picks are right that humans are influencing global warming, but they're wrong to doubt that humans are the dominant cause of the warming over the past six decades.