How We Know Today’s Climate Change Is Not Natural
Last week, the House Committee on Science, Space and Technology, chaired by climate contrarian Lamar Smith, R-Texas, held a hearing on climate science. The hearing featured three scientists who are dubious about the conclusions of the majority of climate scientists, and climate scientist Michael Mann, best known for his “hockey stick graph” of temperatures over the last thousand years illustrating the impact of humans on global warming.
This week, Scott Pruitt, Environmental Protection Agency administrator, who had said that human activity was not the primary contributor to global warming, acknowledged that it plays a role—but stressed the need to figure out exactly how much of one.
Despite the many climate “skeptics” in key positions of power today, 97 percent of working climate scientists agree that the warming of Earth’s climate over the last 100 years is mainly due to human activity that has increased the amount of greenhouse gases in the atmosphere. Why are they so sure?
Earth’s climate has changed naturally over the past 650,000 years, moving in and out of ice ages and warm periods. Changes in climate occur because of alterations in Earth’s energy balance, which result from some kind of external factor or “forcing”—an environmental factor that influences the climate. The ice ages and shifting climate were caused by a combination of changes in solar output, Earth’s orbit, ocean circulation, albedo (the reflectivity of the Earth’s surface) and makeup of the atmosphere (the amounts of carbon dioxide and other greenhouse gases such as water vapor, methane, nitrous oxide and ozone that are present).
Scientists can track these earlier natural changes in climate by examining ice cores drilled from Greenland and Antarctica, which provide evidence about conditions as far back as 800,000 years ago. The ice cores have shown that rising CO2 levels and rising temperatures are closely linked.
Scientists also study tree rings, glaciers, pollen remains, ocean sediments, and changes in the Earth’s orbit around the sun to get a picture of Earth’s climate going back hundreds of thousands of years or more.
Today, CO2 levels are 40 percent higher than they were before the Industrial Revolution began; they have risen from 280 parts per million in the 18th century to over 400 ppm in 2015 and are on track to reach 410 ppm this spring.
In addition, there is much more methane (a greenhouse gas 84 times more potent than CO2 in the short term) in the atmosphere than at any time in the past 800,000 years—two and a half times as much as before the Industrial Revolution. While some methane is emitted naturally from wetlands, sediments, volcanoes and wildfires, the majority of methane emissions come from oil and gas production, livestock farming and landfills.
Global temperatures have risen an average of 1.4˚ F since 1880. Sea ice in the Arctic has thinned and decreased in the last few decades; the Greenland and Antarctic ice sheets are decreasing in mass. The North and South Poles are warming faster than anywhere else on Earth. Glaciers are retreating on mountains all over the world. Spring snow cover in the Northern Hemisphere has decreased over the last 50 years.
The number of record-breaking hot temperatures in the U.S. is on the rise. Oceans are the warmest they have been in a half-century; the top layer is warming about 0.2˚F per decade. The oceans are also 30 percent more acidic than they were at the start of the Industrial Revolution because they are absorbing more CO2. Global sea levels rose an average of 6.7 inches in the last century, and in the last 10 years, have risen almost twice as fast.
Here is how scientists know that the climate change we are experiencing is mainly due to human activity and not a result of natural phenomenon.
Gavin Schmidt, director of National Aeronautics and Space Administration’s Goddard Institute for Space Studies, said that scientists look at a lot of different things at once.
“We have a very, very clear understanding that the amount of heat in the ocean is increasing—the ocean heat content is going up by a lot,” said Schmidt. “That implies that there must be an external change in the radiation budget of the earth—more energy has to be going in than leaving.
“There are a number of ways that can happen, but each of them has a different fingerprint. If the sun were brighter, we would see warming all the way up through the atmosphere from the surface to the stratosphere to the mesosphere. We don’t see this. We see instead warming at the surface, cooling in the stratosphere, cooling in the mesosphere. And that’s a signature of greenhouse gas forcing, it’s not a signature of solar forcing. So we know it’s not solar.”
Moreover, according to the World Radiation Center, the sun’s radiation has not increased since at least 1978 (when satellite monitoring began) though global temperatures over the last 30 years have continued to rise.
In addition, the lower atmosphere (troposphere), which is absorbing the CO2 and expanding as it gets warmer, is pushing the boundary between the troposphere and the stratosphere upwards. If the sun’s radiation were the main factor responsible for Earth’s warming, both atmosphere layers would likely be warming and this would not occur.
Scientists also can distinguish between CO2 molecules that are emitted naturally by plants and animals and those that result from the burning of fossil fuels. Carbon molecules from different sources have different numbers of neutrons in their nuclei; these different versions of molecules are called isotopes. Carbon isotopes derived from burning fossil fuels and deforestation are lighter than those from other sources. Scientists measuring carbon in the atmosphere can see that lighter carbon molecules are increasing, corresponding to the rise in fossil fuel emissions.
Peter de Menocal, dean of science at Columbia University and founding director of Columbia’s Center for Climate and Life, studies deep-sea sediments to understand past climate change.
“Ocean sediments provide a longer term baseline [tens of millions of years] that allows you to compare the past with the present, giving you an idea of how variable ocean temperatures have been before we had thermometers,” said de Menocal. “Over the last 2,000 years, there have been natural climate variations, but they were not especially large…the Medieval Warm period around 1,000 years ago, and the little ice age which was three separate cooling periods lasting a few decades each, beginning around 1300 to around the 1850s. It’s the warming after the 1850s that’s been really remarkable and unique over the last couple of millennia—you can see that in the sediment cores.”
Evidence from ocean sediments, ice cores, tree rings, sedimentary rocks and coral reefs show that the current warming is occurring 10 times faster than it did in the past when Earth emerged from the ice ages, at a rate unprecedented in the last 1,300 years.
To understand this rapid change in climate, scientists look at data sets and climate models to try to reproduce the changes that have already been observed. When scientists input only natural phenomena such as the sun’s intensity, changes in the Earth’s orbit and ocean circulation, the models cannot reproduce the changes that have occurred so far.
“We have independent evidence that says when you put in greenhouse gases, you get the changes that we see,” said Schmidt. “If you don’t put in greenhouse gases, you don’t. And if you put in all the other things people think about—the changes in the earth’s orbit, the ocean circulation changes, El Niño, land use changes, air pollution, smog, ozone depletion—all of those things, none of them actually produce the changes that we see in multiple data sets across multiple areas of the system, all of which have been independently replicated.” In other words, only when the emissions from human activity are included, are the models and data sets able to accurately reproduce the warming in the ocean and the atmosphere that is occurring.
“Today, almost 100 percent [plus or minus 20 percent] of the unusual warmth that we’ve experienced in the last decade is due to greenhouse gas emissions,” said de Menocal.
Findings from NASA’s Goddard Institute for Space Studies show clearly how much natural and manmade factors contribute to global warming.
Climate deniers offer a variety of bases for their skepticism without providing scientific evidence. The most effective thing that the climate denier community has done, however, is to spread the notion of uncertainty about climate change, and use it as an excuse not to take any action.
“It’s been a very effective tactic,” said de Menocal, “in part because the scientific community spends a tremendous amount of effort quantifying that uncertainty. And so we make it plain as day that there are things we’re certain about, and things we’re uncertain about. There are places of debate that exist in the community. That’s the scientific process. … The deniers are not selling a new way of looking at the problem, they’re selling doubt, and it’s very easy to manufacture doubt.”
“They are in total denial of the evidence that there is,” said Schmidt. “When I challenge them to produce evidence for their attributions, all I get is crickets. There’s no actual quantitative evidence that demonstrates anything. … Show me the data, show me your analysis.”
“There are lot of things that we’re absolutely certain about,” said de Menocal. “We’re absolutely certain carbon dioxide is rising in the atmosphere. We’re absolutely certain it’s warming the planet and we’re absolutely certain that it’s acidifying the oceans.”