During his confirmation hearing for secretary of state, Rex Tillerson said “our ability to predict” the effect of increased greenhouse gas concentrations in the atmosphere “is very limited.” That’s not entirely accurate.
While “very limited” is subjective, scientists have differing degrees of confidence when attributing different phenomena to increased atmospheric carbon dioxide and other greenhouse gases.
For example, scientists are nearly certain that increased CO2 levels from the burning of fossil fuels have caused over half of the global temperature increase since 1950. They’re also relatively certain that global warming will lead to a rise in extreme heat waves and sea levels. But they are less certain when linking global warming to increased incidences of wildfires.
Tillerson, the recently retired chairman and chief executive officer of ExxonMobil, is President-elect Donald Trump’s choice for secretary of state. While testifying before the Senate Foreign Relations Committee on Jan. 11, Tillerson acknowledged that climate change is real, saying that “the risk of climate change does exist, and that the consequences of it could be serious enough that action should be taken.”
But he added that, “The type of action seems to be where the largest areas of debate exist in the public discourse.” He then went on to question scientists’ ability to predict the effects of increased greenhouse gas levels.
Tillerson, Jan. 11: The increase in greenhouse gas concentrations in the atmosphere are having an effect. Our ability to predict that effect is very limited.
Scientists’ ability to make predictions based on a particular theory corresponds to the number of times they’ve verified that theory using different lines of evidence: The more verification, the more likely it is that their predictions will turn out to be accurate.
To start, scientists have verified the theory of the greenhouse effect, which says that gases like CO2 trap the sun’s heat, time and again since the physicist Joseph Fourier first proposed it in 1824.
In short, scientists are nearly certain that increased CO2 levels from the burning of fossil fuels will lead to a warmer earth. This is no longer a matter of prediction.
As we have written before, the U.N.’s Intergovernmental Panel on Climate Change’s fifth assessment report that was released in 2013 concluded that it is “extremely likely” that more than half of the observed temperature increase since 1950 is due to human activities.
Scientists also have high confidence that global warming will lead to changes in the climate, including a rise in extreme weather events and sea levels. This is also no longer a matter of prediction.
Take sea level rise: Hundreds of scientists who collaborated on the U.S. Global Change Research Program’s 2016 report have “very high” confidence that global sea level has risen during the past century, estimating about eight inches since 1880. Their evidence comes from 130 years of tide gauge records and 20 years of satellite observations.
The authors also have “medium confidence that global sea level rise will be in the range of 1 to 4 feet by 2100.” However, since scientists have a relative “[l]ack of knowledge” about “how ice sheets in Greenland and Antarctica will react to a warming climate,” some “decision-makers may wish to consider” a range of 8 inches to 6.6 feet by 2100, they add.
Why does sea level rise matter?
The report adds, “Nearly 5 million people in the U.S. live within 4 feet of the local high-tide level,” which means that in “the next several decades, storm surges and high tides could combine with sea level rise and land subsidence to further increase flooding in many of these regions.”
Flooding during “king tides” has already occurred in Florida, Delaware and California, however. King tides are “the highest predicted high tide of the year at a coastal location,” according to the U.S. Environmental Protection Agency. This flooding also will have economic impacts, says the National Oceanic and Atmospheric Administration.
As for extreme weather, the level of certainty scientists have when attributing different kinds of events to global warming comes from the robustness of their climate models and observational records. Their understanding of the physics behind particular events as related to climate change also influences their confidence levels, a 2016 report by the National Academies of the Sciences explains.
Here’s a rule of thumb: The less directly related a type of extreme weather event is to temperature, the less confident scientists are when tying that type of event to human-induced global warming.
For example, the NAS report says scientists have high confidence when linking global warming to an increased likelihood of extreme heat events, which is directly related to temperature.
But scientists have medium confidence when attributing droughts and extreme rainfall to global warming. Heavy rainfall, for example, “is influenced by a moister atmosphere, which is a relatively direct consequence of human-induced warming, though not as direct as the increase in temperature itself,” the report says.
The NAS report adds that tying the frequency and intensity of tropical cyclones, or hurricanes, to global warming is more difficult. Scientists have less data on hurricanes of the past, which limits their ability to make solid predictions. Still, they do have an understanding of the physical mechanisms related to climate change that could bring about these storms.
Recently, new modeling techniques also have enhanced scientists’ confidence when linking cyclone activity to global warming. “Tropical cyclones are projected to become more intense as the climate warms,” the report’s authors write. They add, “There is considerable confidence in this conclusion, as it is found in a wide range of numerical models.”
The 2013 IPCC report also points out that confidence when attributing tropical cyclones to human-caused global warming differs by region. For example, scientists are “virtually certain,” meaning at least 99 percent sure, that there’s been an “increase in the frequency and intensity of the strongest tropical cyclones since the 1970s” in the North Atlantic basin.
However, scientists have relatively low confidence when attributing wildfires to global warming. Numerous factors, including forest management, also play significant roles in increasing the likelihood of these events, which is why scientists have a harder time teasing apart climate change’s contribution. Still, “many studies have linked an increase in wildfires to climate change,” the 2016 NAS report says.
All of these extreme weather changes “will continue to affect human health, water supply, agriculture, transportation, energy, coastal areas, and many other sectors of society, with increasingly adverse impacts on the American economy and quality of life,” experts on the U.S. Global Change research program concluded.
To be clear, Tillerson’s statement that scientists’ ability to predict the future impacts of increased greenhouse gas levels is “very limited” is subjective. It’s also worth noting that last October he said he supported a revenue-neutral carbon tax. This term refers to a tax directly linked to greenhouse gas emissions that doesn’t increase federal revenue because it’s offset by other tax reductions. The World Bank calls a carbon tax “an essential part of the solution” to combating climate change. Scientific predictions are also part of the solution.
Editor’s Note: SciCheck is made possible by a grant from the Stanton Foundation.