Climate change could be a lot worse for America than you thought. That’s one of the messages from deep inside the 600-page final draft of the Congressionally-mandated National Climate Assessment (NCA) that was leaked to the New York Times this week.
If America stays on its current emissions path — often referred to as business-as-usual, or for all intents and purposes, the path President Donald Trump seems determined to follow — the assessment identifies four extreme outcomes that could take a substantial toll on people across the country.
- Sea level rise of 1 foot per decade after 2050, and 2 feet per decade after 2100.
- Devastating drops in soil moisture across most of the country, including our breadbaskets.
- Weakening of the Atlantic Ocean’s circulation, which would speed up warming and sea level rise.
- What the report refers to as “Potential surprises: Compound extremes and tipping elements.”
In essence, the study lays out the choice between manageable warming (if we embrace and then keep strengthening the Paris climate agreement) and unmanageable catastrophe (if we pursue Trump’s policies of undoing global and U.S. climate action).
First, in terms of sea level rise, the report can’t rule out rise 8 feet (or more) of sea level rise by century’s end (the “extreme” scenario). But then, the Arctic sees upwards of 18°F warming in the 2071-2100 timeframe, so melting of the Greenland ice sheet will be off the charts.
Even in the “intermediate-high” and “high” scenarios — what one might think of as the plausible worst-case scenarios on the business-as-usual path — sea levels rise 4.9 and 6.6 feet respectively.
But the report also focuses on a topic that gets a lot less attention: the rate of sea level rise in the coming decades. In the “high” scenario, seas are rising 8 inches per decade by 2050, and 14 inches per decade by 2090. That rise continues to accelerate until it reaches a rate of 2 feet per decade early in the next century.
How exactly do you adapt to such rapid and accelerating sea level rise? How do you design a port city for such a world? How rapidly do you abandon places that you know will be repeatedly inundated by the combination of sea level rise and storm surge in the coming decades?
The second key impact identified in the report is that it predicts most of our country would see serious to devastating drops in soil moisture most of the year. Here are the key projections for change in soil moisture by 2100, again in the “higher emissions” (business as usual) case, the one the Trump administration’s policies would lead to.
This would be the “new normal” climate, a near permanent state of drought over large segments of our country, including much of the California and Great Plains breadbaskets. In such a world, we would routinely suffer megadroughts as intense as the 1930s Dust Bowl, but lasting many decades.
Also, as the maps show, soil conditions will be even worse south of the border, home to hundreds of millions of people in 2100. We’d be looking at the conditions for a massive refugee crisis and failed states — particularly when you consider the large fraction of people in Central America who live along the coasts, an area that would be subject to devastating sea-level rise.
Third, global warming appears to be weakening a crucial ocean circulation in the North Atlantic, the Gulf Stream system, more officially known as the Atlantic Meridional Overturning Circulation (AMOC). This slowdown was the basis of the very unscientific climate thriller, The Day After Tomorrow. And while there is exactly zero chance this shutdown will cause an Ice Age for us, “a slowing or collapse of the AMOC would have several consequences for the United States,” the NCA authors explain.
“A decrease in AMOC strength is probable,” though a complete collapse this century is considered very unlikely. Such a decrease “would accelerate sea level rise off the northeastern United States, while a full collapse could result in as much as approximately 1.6 feet of regional sea level rise.” This extra rise in East Coast sea levels would be on top of whatever multi-foot sea level rise the entire world sees. An AMOC slowdown would reduce regional warming a bit, but “would also lead to a reduction of ocean carbon dioxide uptake, and thus an acceleration of global-scale warming.”
The fourth major extreme consequence of the business-as-usual emissions path is multi-faceted; the scientists’ devote their entire final chapter to “Potential surprises: Compound extremes and tipping elements.” The authors want policymakers and others to understand that the impacts of climate change could be even worse, and in ways we can’t be entirely certain about.
“Positive feedbacks (self-reinforcing cycles) within the climate system have the potential to accelerate human-induced climate change and even shift the Earth’s climate system, in part or in whole, into new states that are very different from those experienced in the recent past,” warn the authors. These include states “with greatly diminished ice sheets or different large-scale patterns of atmosphere or ocean circulation).”
Also, nobody has been really studying what happens when you have multiple impacts at the same time. “The physical and socioeconomic impacts of compound extreme events (such as simultaneous heat and drought, wildfires associated with hot and dry conditions, or flooding associated with high precipitation on top of snow or waterlogged ground) can be greater than the sum of the parts,” they explain. But “few analyses consider the spatial or temporal correlation between extreme events.”
What happens, for instance, when you have extreme drought in the central parts of your country, and extreme sea level rise and storm surge in the coastal areas? That’s what all of North America faces, but it hardly ever gets studied.
Now mix in a whole bunch of dangerous tipping points, many of which can be found in this figure:
For instance, the authors note that “climate model experiments suggest that warming will reduce the threshold needed to trigger extremely strong El Niño and La Niña events.” Another possible tipping point is “Arctic sea ice, which may exhibit abrupt state shifts into summer ice-free or year-round ice-free states.”
The great ice sheets are yet another tipping point. In particular, the Antarctic ice sheet is unstable, it is melting from below, and “the amount of ice that sits on bedrock below sea level is enough to raise global mean sea level by 75.5 feet.”
The scientists point out that while basic climate processes can be well quantified, climate models “do not include all of the processes that can contribute to feedbacks, compound extreme events, and abrupt and/or irreversible changes.” That leads to the worrisome conclusion that “for this reason, future changes outside the range projected by climate models cannot be ruled out.”
Finally the authors want policymakers to know that the impacts stemming from unchecked climate change are more likely to be worse than expected: “Moreover, the systematic tendency of climate models to underestimate temperature change during warm paleoclimates suggests that climate models are more likely to underestimate than to overestimate the amount of long-term future change.”
The plausible worst-case scenario is unpleasant to think about. But it would be unimaginably worse for our children and billions of people around the globe to actually experience. The best way to make sure that nightmare never happens is to slash carbon pollution now.