The latest report from the world’s climate scientists has made clear the size of the challenge if the world is to stay below the global warming limit hoped for in the Paris climate agreement. Unfortunately, with current trends we are likely to cross this threshold within the next two decades because we are already two-thirds of the way there.
But how do we know what is driving these climate trends? It comes down to the same kind of detective work that typifies a crime scene investigation, only here we are dealing with a case that encompasses the whole world. Let me give you my view, which does not necessarily represent the position of NASA or the federal government.
For the past 100 years we have documented good, independently confirmed observations of change at the surface of the planet, and for the past 40 years satellites and comprehensive measuring efforts have provided a much fuller view of changes throughout the earth system. These observations show clearly that among other things, the surface of the planet has warmed, the upper atmosphere has cooled, the oceans are gaining an enormous amount of heat, sea level is rising, Arctic ice has greatly receded and glaciers around the world are in retreat.
Scientists have no shortage of suspects for the causes of climate change. Over the 4.5-billion-year history of the planet, almost anything that might have happened has happened, sometimes many times over. The sun has brightened and sometimes dimmed, massive volcanic eruptions have occurred, the planet’s orbit has wobbled, the continents have moved, the land surface has been remade, the composition of the atmosphere and its ability to trap and reflect solar and terrestrial radiation has altered, ocean circulations have sputtered and stopped, and we have been struck, at least once, by a mass-extinction-inducing asteroid.
Each of these events left a unique fingerprint of change on the climate system, with impacts reaching from the upper atmosphere 30 miles high to the deep ocean four miles down, from the tropics to the poles, and in the sediments laid down in the geological record. To track down the culprit of any one specific climate change involves piecing together the contemporaneous fingerprints and tracking them back to the plausible causes.
For changes since the beginning of the 20th century we don’t need to worry about asteroids or moving continents, but we know there have been natural changes in earth’s orbit, variations in the sun’s brightness and volcanic activity. There are some new suspects too. Human activity has deforested, replanted and irrigated large areas of land, added pollution to the skies, depleted the ozone layer and, yes, changed the concentrations of key greenhouse gases in the atmosphere. Carbon dioxide levels are up 40 percent, methane has more than doubled, and we’ve introduced some purely synthetic compounds that are many times more potent than either of those but are fortunately present in lower, but growing, concentrations.
Like forensic detectives, climate scientists have developed a new array of tools in recent decades designed to skillfully calculate what the fingerprints of these changes look like, and more important, how they differ from one another. It turns out that increases in solar activity produce warming throughout the atmosphere, while carbon dioxide increases cooling in the upper atmosphere and warms the surface. Variations in ocean circulation distribute heat, while changes in the sun or in greenhouse gases change the total heat amount in the system. Air pollution, volcanoes and irrigation all cool the climate, while rising greenhouse gases warm it. Ozone depletion has increased the speed of the winds around Antarctica, affecting ocean circulation and sea ice.
But even taking into account uncertainties in the amount of air pollution in the 19th century or in estimating global temperatures through time, scientists have concluded that the current warmth is impossible to explain without human contributions. It is on a par with the likelihood that a DNA match at a crime scene is purely coincidental. Moreover, when we include the multiplicity of human effects, we match them with the observed trends at the surface, in the Arctic, in the ocean and aloft. The dominant factor that emerges is the rise in greenhouse gases, which we know comes mainly from the burning of fossil fuels and deforestation.
Even more convincingly, these trends aren’t just being attributed in hindsight. The rate of surface warming was predicted in the 1980s, the cooling in the upper atmosphere was forecast in a 1967 scientific paper, and specific measurements from space indicate that the total greenhouse effect has been enhanced exactly as theory would predict.
When this is all put together, the conclusions are inescapable: Without human activities the planet would not have warmed over the past century. When scientists include all of the effects that humans have had on the climate system, they can match them with these many independent and varied observations. Our best assessment is therefore that humans, at least the ones responsible for the bulk of carbon dioxide emissions, have been responsible for all of the recent trends in global temperatures.
The forensics have spoken, and we are to blame.