It is obvious that all weather now occurs in a human-altered climate. Thus, the question is not if a weather event has been caused by climate change, but rather which features of that event are attributable to climate change. Attribution is a rapidly growing branch of climate science that is showing us how features of extreme weather, floods, droughts, and fires are likely connected to climate change. As much as 30 percent of the mortality associated with the European heat wave of 2003 is attributable to climate warming.
Hurricane Dorian is indicative of how the features of cyclones can be attributed to anthropogenic climate change. Such attribution also applies to aspects of hurricanes Harvey, Florence, Irma, Michael, and superstorm Sandy. As reported last year by the National Science Foundation hurricanes worldwide are becoming slower, wetter, and stronger. The science about hurricanes is becoming increasingly definitive. It seems clear that such storms will be part of our future on the Atlantic coast of the United States.
The slowdown is attributed to the general weakening of atmospheric circulation in the tropics, where hurricanes form, and in the mid-latitudes where hurricanes often make landfall. The reduced translational speed has occurred worldwide and may be associated with the general weakening of the temperature gradient between the Arctic and the equator. In effect, the slowdown is being driven by Arctic Amplification, which is the more rapid and extensive warming of the regions closer to the north pole. Cyclones in most ocean basins have slowed down 17 percent relative to 1944–2017. Dorian’s forward speed was about 1 mile per hour when it effectively stalled over the Bahamas causing devastation of Abaco and Grand Bahama.
This slow-down is partly to blame for hurricanes becoming wetter. Slow-moving storms can acquire more water as they move over hot oceans. Moreover, the atmosphere holds about 7 percent more water vapor with every 1˚C warming. The atmosphere has warmed about 1˚C since preindustrial times and there is more energy and water vapor aloft. This energy is dissipated as extreme weather. The enormous amount of precipitation carried by Harvey and Florence is testimony to this reality.
Of course, the general strengthening of hurricanes is directly related to warming oceans and sea surface temperatures. It is arguable that our oceans are warming far faster than they have since the Pliocene, 3 million years ago, which was the last time CO2 concentrations exceeded 400 ppm and sea level rise eventually exceeded 6 meters. More than 90 percent of anthropogenic warming has occurred in the ocean. A study published in January 2019 showed that the warming of the oceans has been 30 percent greater than that reported in the latest IPCC report (2014).
Although there have been five Category 5 storms in the Atlantic in four years, an unresolved question is whether hurricanes will become more frequent as the Anthropocene progresses. For many of us who live near the Atlantic coast the answer to this seems obvious, but only time will reveal if the recent pattern is statistically robust. Many hurricanes before 1900 likely went unrecorded. The satellite era allowed us to track storms only since the mid-1960s and we have been using the Saffir-Simpson index for rating storm power since 1974.
An additional feature that is likely attributable to climate change is the rate of hurricane intensification. Wind shear can diminish hurricane power and speed of intensification. A recent paper showed that because of climate change wind shear is diminished as hurricanes enter the northern Caribbean and approach the Atlantic coast. This has resulted in an increase in the speed of intensification of recent hurricanes in the Atlantic. Such rapid intensification was a feature of hurricane Michael, which went ashore as a category 4 and utterly devastated Mexico Beach, Florida.
Given the reality of our future it is critical that we assess how we can proactively adapt to reduce the impact of extreme weather events. Although state and federal officials are responsible for warning us and providing relief after devastation has been wrought (assuming funds are not depleted), it is communities that have the greatest ability to take action to reduce the loss of property and life. The community is where the impacts of climate and ecosystem disruption are experienced, and it will be the front line of adaptation to these impacts. Such proactive adaptation is far less expensive and disruptive than reactive adaptation.
Communities can do many things to reduce the impacts of extreme events. For hurricanes, it is essential that we move homes and businesses out of harm’s way. Storm surge is amplified by sea level rise as exemplified by the surge from superstorm Sandy which hammered parts of the Atlantic coast of New York. Analyses of options in response to sea level rise result in the inescapable conclusion that strategic retreat from the coast is necessary. We should begin retreat now rather than deal with the enormous expense of cleaning up the mess after disaster strikes.
Sea walls and pumps won’t do the job and they are certainly not cost effective. The recent installation of pumps to counteract coastal flooding in Miami will buy precious little time and is probably a waste of money. Insurers are quietly withdrawing from insuring coastal and near-coast properties, especially for the Florida Atlantic coast. A study published last week in Nature Communications showed that the compound effects of sea level rise and increased hurricane power by the end of the century will result in the historical 100-year flood level occurring annually in New England and the mid-Atlantic coast. In the southeast Atlantic and Gulf the return rate of such floods will be every 1–30 years.
Perhaps as important as controlling where homes and businesses are built is the human infrastructure and scenario planning necessary to protect citizens during extreme events. Studies of Sarasota Bay provide a daunting scenario in which a category 4 storm sweeps ashore and devastates the high rises close to the bay. Many of the properties in this region are occupied by an aging population without access to independent transportation. The city and the county must continually revise plans to evacuate this population when a hurricane becomes imminent. As Sarasota County officials will tell you, this will be a herculean task that will require extensive coordination among overlapping jurisdictions in the region. Following the devastation of Dorian more than 70,000 are homeless and thousands from the Bahamas have become climate refugees.
Community organizations and non-governmental organizations must develop the capacity to coordinate proactive adaptation and plans with city and county agencies. Such planning is indispensable when an emergency arises. To be sure, our warning systems now provide ample lead time for a variety of coordinated efforts among organizations and government. Even though Dorian failed to make landfall along the Florida coast, the preparation that ensued from the warnings would have been indispensable had such landfall occurred.
The financial imperative of development must not be allowed to hijack realistic plans for proactive adaptation. An example of this is the long battle between citizens and deep-pocket developers who wanted to build Green Diamond, a billion-dollar development on the Congaree River floodplain in South Carolina. Fortunately, a coordinated effort by an unlikely coalition of citizen groups and concerned government officials stopped this development just before there was massive flooding in this region. In Florida a similar nightmare scenario has played out as developers have built with reckless abandon in several regions near the coast. We must make plans based on future projections, not on historical precedent.
Because of pressure from financial interests and politicians interested in short-term gain, the US is behind other countries in efforts at proactive adaptation. As reported in Science, bans on rebuilding in the path of storms and floods have been implemented in Australia, Germany, Japan, New Zealand, and the Philippines. Colombia, India, and Mozambique require resettlement after disaster to pre-identified safe locations. Governments have engaged in acquisition of vulnerable property in Colombia, Japan, Netherlands, New Zealand, and somewhat in the United States. Such acquisition is demonstrably less expensive than paying the bill after tragedy strikes. Some communities in Australia, China, Fiji, India, and Vietnam have been completely relocated.
Although there continues to be rather meaningless academic debate about the use of the term Anthropocene, there can be little doubt human actions over the last two centuries have profoundly altered the Earth. Future generations likely will be affected for millennia. If we stop all fossil emissions the return of the climate to something resembling the putative normal conditions of the 20th century will require hundreds to thousands of years through natural processes. Today you have experienced the most stable climate that you will ever experience for the remainder of your lifetime. Throughout my teaching I ask my students to accept the reality that active management of our climate and biosphere will be required for so long as civilization is to exist on Earth. We best get to it before the damages are so great that we can no longer stabilize the Earth System.