The Line: Scientists cannot precisely measure climate change nor the impact of human activity on climate change.

The Party: Republican

In the past, some Republicans have denied that global warming and climate change are occurring — calling the former a "hoax" and the latter "pseudoscientific." In recent months, a number of President Donald Trump's cabinet members have taken a different stance: They acknowledge that the climate is changing and humans have contributed, but they've said that scientists can't measure or don't understand human impact precisely.

But scientists can measure that impact with varying levels of certainty and precision.

During his confirmation hearing on Jan. 18, Environmental Protection Agency Administrator Scott Pruitt said, "Science tells us that the climate is changing and that human activity in some manner impacts that change," adding, "The ability to measure with precision the degree and extent of that impact, and what to do about it, are subject to continuing debate and dialogue, and well it should be."

Pruitt repeated this claim on Feb. 28 in an interview with CNN'S Wolf Blitzer and on Feb. 25 at the Conservative Political Action Conference. For example, he told Blitzer "we know that there is a warming or — a warming of the planet; climate change is occurring; and there's some human contribution to that or human activity that contributes to that. How to measure that precisely is very challenging."

Interior Secretary Ryan Zinke, Health and Human Services Secretary Tom Price and Secretary of State Rex Tillerson have made similar remarks.

At his confirmation hearing on Jan. 11, Tillerson said, "The increase in the greenhouse gas concentrations in the atmosphere are having an effect. Our ability to predict that effect is very limited."

While "precision" and "very limited" are subjective terms, scientists have differing degrees of confidence when linking different phenomena to increased atmospheric greenhouse gases from the burning of fossil fuels.

Scientists' confidence in a particular theory corresponds to the number of times they've verified that theory using different lines of evidence: The more verification, the more confidence, the more likely it is that scientific estimations and predictions are and will be accurate.

Confidence in one theory can also rely, in part, on confidence in another theory. In this case, the theory of climate change rests upon the accuracy of the theory of global warming, which depends on the theory of the greenhouse effect.

From the Greenhouse Effect to Global Warming

Scientists have the most confidence in the greenhouse effect — that carbon dioxide, methane and other greenhouse gases trap the sun's heat — because they've verified it countless times since the physicist Joseph Fourier first proposed it in 1824.

In fact, the design of heat-seeking missiles relies upon a precise understanding of the greenhouse effect.

Because of their confidence in the greenhouse effect, scientists also are nearly certain that increased atmospheric greenhouse gas concentration from the burning of fossil fuels has led and will continue to lead to a warmer Earth.

According to the U.N.'s Intergovernmental Panel on Climate Change's 2013 assessment report, scientists are "virtually certain" (99 percent to 100 percent confident) that natural climatic variability "alone cannot account for the observed global warming since 1951." The report concludes that it is "extremely likely" (95 percent to 100 percent confident) that more than half of the observed temperature increase since 1950 is due to human activities.

This graph shows that the increase in average global temperature paralleled an increase in carbon dioxide concentration in the atmosphere.

Since 2013, the trend has only continued.

After independently analyzing temperature records dating back to the 1800s, NASA, the National Oceanographic and Atmospheric Administration and the U.K.'s Met Office all concluded that 2015 and 2016 were likely the two hottest years on record.

From Global Warming to Climate Change

Multiple lines of evidence also suggest that global warming has changed and will continue to change climate and weather patterns around the world, namely by increasing sea levels and increasing the likelihood of extreme weather events. However, scientists have different confidence levels when attributing these various phenomena to global warming.

Take sea level rise: Hundreds of scientists who collaborated on the U.S. Global Change Research Program's 2014 report have "very high" confidence that global sea level has risen about 8 inches since 1880.

"Very high" confidence means evidence comes from "multiple sources" that obtained "consistent results" using "well documented and accepted methods," which led to "high consensus" within the scientific community.

The authors also have "medium confidence that global sea level rise will be in the range of 1 to 4 feet by 2100" (see image below).

"Medium" confidence means evidence comes from "a few sources" that used "emerging" methods and obtained "limited consistency," which has led to "competing schools of thought" within the scientific community.

Credit: J. Willis, Jet Propulsion Laboratory. This figure appears in chapter 2 of the Climate Change Impacts in the United States: The Third National Climate Assessment report (2014).

To be clear, this doesn't mean that global sea level won't continue to rise — it will.

Rather, scientists have some uncertainty about the when and the how much of sea level rise. Uncertainty results from difficulty in predicting "how ice sheets in Greenland and Antarctica will react to a warming climate," the authors explain. Given this uncertainty, "Some decision-makers may wish to consider a broader range of scenarios such as 8 inches or 6.6 feet by 2100 in the context of risk-based analysis," the authors add.

But in specific regions, the report authors had greater confidence. For example, the Southeast is particularly vulnerable to sea level rise. "New Orleans (with roughly half of its population living below sea level), Miami, Tampa, Charleston, and Virginia Beach are among those most at risk," they write.

Since sea level "is expected to continue to rise for several centuries, even if greenhouse gas emissions are stabilized," the report's authors have "very high" confidence that "[s]ea level rise poses widespread and continuing threats to both natural and built environments and to the regional economy" of the Southeast U.S.

Measuring and Predicting Extreme Weather

The level of certainty scientists have when attributing different kinds of extreme weather 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.

The NAS report authors have high confidence when linking global warming to an increased likelihood of extreme heat events, which is directly related to temperature.

Extreme heat also will impact regions differently.

For example, the U.S. Global Change Research Program's 2014 report states that "the majority of Maryland and Delaware, and southwestern West Virginia and New Jersey, are projected by mid-century to experience many more days per year above 90°F compared to the end of last century under continued increases in emissions."

The area around Washington, D.C., in particular is expected to see 60 to 70 days a year on average reaching 90 degrees Fahrenheit between 2041 and 2070, compared with 20 to 30 days above 90 F between 1971 and 2000 — and that's for the lower emissions scenario (see image below).

This image shows the historical and projected number of days per year over 90°F in the northeast U.S. Credit: NOAA NCDC / CICS-NC. This figure appears in chapter 16 of the Climate Change Impacts in the United States: The Third National Climate Assessment report (2014).

According to the 2016 NAS report, scientists have medium confidence when attributing increased likelihood of droughts and extreme rainfall to global warming, in part, because those events are less directly related to temperature increases.

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.

When it comes to specific regions, the northeast U.S. has "experienced a greater recent increase in extreme precipitation than any other region in the United States," the 2014 Global Change report authors write.

The 2016 NAS report states that tying increased frequency and intensity of tropical cyclones, or hurricanes, to global warming is more difficult than other extreme weather events. Why? 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 that could bring about these storms. And recently, new modeling techniques have enhanced scientists' confidence when linking hurricane activity to global warming.

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 Ocean, which hugs the East Coast.

Lastly, scientists have relatively low confidence when attributing increased likelihood of wildfires to global warming because numerous factors, including forest management, also play significant roles in increasing the likelihood of these events. Still, "many studies" have linked increased wildfires to global warming, the 2016 NAS report says.

Scientists also have more confidence when linking global warming to the increased likelihood of wildfires in particular regions. For example, the 2014 Global Change report authors have "high" confidence that "increased warming" and climate change-related "drought, and insect outbreaks" have "increased wildfires" in the southwest U.S. "Fire models project more wildfire and increased risks to communities across extensive areas" in this region, they add.

"High" confidence means that evidence comes from "several sources" that show "some consistency," but "methods vary," which has led to "medium consensus" in the scientific community.

Overall, the level of precision and confidence scientists have when linking human activity to different changes in the global and local climates vary. Sometimes scientists can be very precise and have very high confidence in their conclusions. In other cases, it's more difficult to be precise, which corresponds to lower levels of confidence in their estimations and predictions.

Republicans who have said scientists cannot precisely measure climate change or the impact of human activity on climate change:

EPA Administrator Scott Pruitt, Feb. 28:
— I mean, as I indicated in my hearing, Wolf, I mean, we know that there is a warming or — a warming of the planet; climate change is occurring; and there’s some human contribution to that or human activity that contributes to that. How to measure that precisely is very challenging. (Source: CNN interview.)

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