In the last couple of weeks, we saw the nomination hearings for Rex Tillerson, Scott Pruitt, Ryan Zinke, and Rick Perry each of whom will exert considerable influence over climate and energy policy in the Trump Administration, and who were accordingly asked about their opinions on climate change.

None of them seemed to consider it a policy priority. They did, however, uniformly acknowledge the reality of climate change. While they generally accepted the human impact on the climate, they also cited the potential for some natural changes.

This represents a new stance for GOP leadership, as it is both a retreat from the silence of the 2016 Republican Platform, and a shift from the “I’m not a scientist” position held just a year or so ago. Dig a couple years further back, and Rick Perry even decried climate hysteria because he thought it was based on fraudulent science.

This shift even positions Trump’s cabinet somewhat ahead of the Republican caucus in the Senate. Recall that in 2015, only 15 Republican Senators could muster a ‘yay’ vote to acknowledge that humans contribute in some part to climate change, while all but one Republican Senator agreed it was not a hoax. Uniformly, this year’s nominees said that they did not believe climate change is a hoax, and that human activity plays some role in driving it.

Not everyone sees this as a victory or a particularly new position. Commenting for the Christian Science Monitor, Susan Joy Hassol described this move as “a return to the George W. Bush Administration,” when the official position was that the science was rather uncertain and we should have more research before doing anything hasty or harming the economy.

Key to this hesitation is the idea that while the climate is changing, there is room for debate about how much of it is caused by humans versus natural changes. The difference between now and the Bush years, however, is that our scientific understanding of the human impact on climate stands on firmer ground.

With a decade and a half of research and three IPCC assessment reports (TAR, AR4, AR5) since Bush 43 took office, scientific conclusions about the human influence on warming have been repeatedly tested and have grown more confident. Compare the standing wisdom at the start of the Bush Administration from the IPCCs 1995 Second Assessment Report, “[t]he balance of evidence suggests a discernible human influence on global climate,” to the conclusion from the last IPCC summary for policymakers:

It is extremely likely that 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 difference is not purely rhetorical either, but reflects the advances in understanding that have come from continued research.

The following figure, put together by Dana Nuccatelli for Skeptical Science, illustrates that multiple studies have contributed to this understanding over the last 16 years. It shows the percentage of total warming over the last half century or so (different studies start and end at different years, but global warming over that period was roughly 1 °F) assigned to human factors on the left, and the percentage assigned to natural factors of the right.


Separating human and natural causes of observed warming requires a climate model. We can’t go back in time to recreate the world without the increase in CO2 emissions as a counter-factual, so scientists do so in a model study. Such studies incorporate factors that we know influence the climate over decades, and are entirely natural and beyond human control, such as cycles in solar intensity and volcanic eruptions. If you add them together to arrive at a full description of so-called natural climate change, you find that they are unable to recreate anything that looks like the warming that has been observed. That’s the bars on the right in the chart.

Instead, only after they account for human influences (mostly increasing greenhouse gases), can scientists recreate the observed warming. That’s the bars of the left.

No measurement is as precise as scientists would like, and no climate model can be constructed perfectly, so there is room for error in how much warming there was, what the different agents of climate change were doing over that period, and how the climate responds to those different agents of change. Taking those uncertainties into account is what gives you something like what the IPCC reported in its last assessment report, that it is extremely likely that human influences caused most of the observed warming.

In this context, that means the IPCC’s latest authors judge that the probability natural causes were responsible for more than 50% of the warming was very small (less than 5 percent, or 1-in-20). There might be room for debate about whether that very small should be 1 percent, 5 percent, or 10 percent, and diverse authors have pointed out how the IPCC’s reasoning might be overconfident (even within the IPCC itself, c.f Box 2.2 on pdf page 24). But from a fair reading of the standing literature, there is vanishingly little reason to believe that warming from the last 50-65 years is mostly natural.

Some experts dispute the conclusion that most of the temperature increase over the last 50-65 years can be chalked up to human activity. But there is hardly uniformity in the point of criticism presented by each. Appeals to changes in cosmic rays or internal variability all run up against a fairly strong body of evidence which indicates  they have had minimal impact on recent trends in global temperature.

The global mean temperature is an entirely remote measure of climate change. No one feels it or sees it. So it is of much greater interest to know what climate changes are linked conclusively to human-caused global warming. Since little else in climate is as well observed as surface temperature, it can be challenging to identify the human signal in other parts of the system with the same confidence. Although once the human climate impact is settled, other changes that are linked by theory and climate models to temperature increases, like sea level rise, show signs of human influence as well.

On regional scales, initial research shows there is much more scientific ambiguity when looking at measures of the human climate impact, especially when considering trends in precipitation. It is harder to find a human signal when observational records are short, local records are noisy, and what signal to expect from global warming is still under debate (which is especially the case for precipitation).

The stage on which nomination hearings take place makes it hard to have a nuanced discussion of the different ways that human influence plays out on the climate. It creates an environment where it is easy to say something true, but not particularly informative. For instance, “the climate is changing but we are not sure how much human influence is contributing,” might be totally true for precipitation trends, but for global temperature it is a major stretch.

I don’t know that any of the nominees had those nuances in mind when they described their views on climate change. While their acknowledgement of a human influence is an improvement, a close read of the science says they have further to go.

Image by Robert Jones from Pixabay