Scientists have known for decades that human-caused greenhouse-gas emissions have been warming the planet. But they have been reluctant to attribute any particular extreme weather event to anthropogenic climate change.
That these links exist seems plausible because of the effects of increasing temperatures on the Earth's water and carbon cycles, polar sea ice, coral reefs, and oceanic- and atmospheric-circulation patterns. But scientists demand a very high standard of proof that a human fingerprint is present before making any claims.
Thus, although 2017 set the all-time record for the most billion-dollar weather disasters in U.S. history, scientists continued to speak in terms of a range of probabilities that fossil-fuel use by humans caused or influenced the severity of these disasters.
Nevertheless, climate science has advanced considerably in the past few decades. The predictive power of climate models continues to improve, providing more realistic representations of Earth's climate system.
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A related advance has been in the science of event attribution, which seeks to determine whether individual severe weather events that have already occurred may have been caused or exacerbated by human activities.
Models predicting Earth's future climate based on fossil-fuel emissions have been around for a long time - long enough, in fact, that data from recent weather events may now be used to evaluate the predictive value of specific models.
For example, researchers at the Carnegie Institution for Science have back-tested various climate models with recent climate and weather observations. Their results show that many models appear to be underestimating future warming and that models predicting the least-severe outcomes may be dismissed. They also concluded that, in order to stabilize Earth's temperature, it will be necessary to reduce emissions by more than previously supposed.
In the field of event attribution, Princeton University and the National Oceanic and Atmospheric Administration accurately predicted that the burning of fossil fuels would lead to an increase in destructive cyclones in the Arabian Sea by 2015. This marks one of the first instances that modeled climate predictions have been verified by recorded weather events.
Other examples abound. The annual report by a scientific panel of the American Meteorological Society separates human-caused climate change from what might be expected from natural variability. For the first time in the report's history, the panel said that some events of 2016 could not have occurred without anthropogenic warming. These events included record heat in Asia and marine hot spots in the Gulf of Alaska, in the Bering Sea, and near the coast of northern Australia.
At Stanford University, Noah Diffenbaugh and his colleagues recently concluded that human-caused warming has increased drought risk in California. Diffenbaugh stated, "Our results suggest that the world isn't quite at the point where every record hot event has a detectable human fingerprint, but we are getting close."
Diffenbaugh and his co-workers also said their work indicates a high probability that human activities affected heat waves in Russia, extreme dry events in the tropics, and the 40 percent decline in Arctic sea ice that has occurred over the past three decades.
Regarding Hurricane Harvey, new research from Lawrence Berkeley National Laboratory and the Netherlands Meteorological Institute found that anthropogenic climate change made the record three-day rainfall that fell over Houston during the hurricane roughly three times more likely and 15 percent more intense than a similar storm occurring in the early 1900s.
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These authors also concluded that even if the goals of the Paris Climate Agreement were met and warming was limited to 3.6 degrees Fahrenheit, the probability of a Harvey-type event would increase an additional threefold. But if we fail to meet that target, the increase of frequency and intensity of storms in the Gulf Coast region could be much higher.
Higher-resolution climate models also are being developed. The University of Illinois and Argonne National Laboratory scientists have developed models that may help policymakers at the local level. These models can account for local variables like bodies of water and mountains. For the Midwest in general, the models indicate that failure to reduce emissions will result in more frequent and serious heat waves that will damage agriculture and human health.
In November 2017, more than 15,000 scientists from around the globe issued a letter warning that human destruction of the natural world will lead to misery and an irretrievably mutilated planet. Politicians may expunge all references to climate change on government websites, but science continues to advance - and Earth gets the last say.
David Gerhart of Duluth has a doctorate in aquatic ecology from Cornell University and has published and reviewed manuscripts for scientific journals in the fields of ecology and biochemistry. This commentary was reviewed and edited by Byron Steinman, a climatologist and assistant professor at the Large Lakes Observatory at the University of Minnesota Duluth, before it was submitted to and edited by the News Tribune.
