On the afternoon of Black Friday, the Trump administration stealthily rolled out the voluminous National Climate Assessment, which describes — region by region across the United States — how global warming is affecting our weather, our communities and the environment. And it details how these warming-induced changes will intensify in the coming decades.

To understand what the report means for Washingtonians, the report’s chapters devoted to the Northeast and Southeast United States are instructive and revealing. They confirm many of the trends we have seen as Washington’s weather has shifted in recent years.

The city is heating up, downpours are intensifying, and our snow season is shrinking. In coastal areas, from the tidal Potomac to the Atlantic beaches, water levels are rising and more frequently inundating the shore. These changes, the report warns, will accelerate into the future if greenhouse gas emissions continue unchecked.

95 degrees becomes ‘commonplace’

Washington has seen steep temperature increases in recent decades, and this is just the beginning. The National Climate Assessment projects substantially more warming in the coming decades. Depending on whether greenhouse gas emissions slow or continue business as usual, temperatures are expected to rise between 4 and 5 degrees by 2050 compared with 1975 to 2005.

The summer heat is predicted to become scorching. Think of Washington’s most brutally hot summers now. They will become the new normal later this century if emissions are not curbed. Under the higher-emissions scenario, summer days above 95 and nights above 75 “become commonplace,” the report projects.

While summers become more extreme, the harshness of winter is expected to ease. The report projects a “shorter and less pronounced cold season” and a “substantial” decrease in the frequency of freezing temperatures.

Since 2010, Washington’s observed temperature extremes appear to fit in well with these projections. Washington has witnessed:

  • Its warmest three years on record (2012, 2017 and 2016).
  • Its two warmest springs (2012 and 2010).
  • Its hottest four summers (2010, 2011, 2016 and 2012) and the most days on record with high temperatures of at least 80 degrees (136 days in 2018) and 90 degrees (67 days in 2010).
  • Its warmest fall (2016).
  • Its third-warmest winter (2012).
  • Its warmest February, March, April, May, June, July and December.
  • Its second-longest freeze-free period (2010), its earliest last freeze (February 2010) and third-latest first freeze (December 2011).

More-extreme rainfall

Washington’s heaviest rainfall has trended more extreme in recent decades, and the assessment projects that this will continue.

“The recent dominant trend in precipitation throughout the Northeast has been towards increases in rainfall intensity,” the report says. “Further increases in rainfall intensity are expected, with increases in precipitation expected during the winter and spring with little change in the summer.”

If emissions are not reduced, the report projects a doubling in the number of heavy-rain events by the end of the century, which will increase the frequency in flooding.

In a report published in 2017, Climate Central, a nonprofit science communication firm in Princeton, N.J., found that Washington was already seeing increases in the frequency of moderate-to-heavy rain events of at least 0.5, 1 and 2 inches.

Perhaps not coincidentally, Washington is on pace to notch its wettest year on record in 2018 and has already seen the most days (22) with at least one inch of rain in a calendar year.

And how can we forget Ellicott City, which just endured two “1 in 1,000″ rain events in two years?

‘A shorter snow season'

Washington has seen its average snowfall sink from about 21 inches in the late 1800s to about 15 inches today. This slide should continue. The Washington region is already one in which temperatures are often only marginally cold enough for snow and, as temperatures rise, rain is likely to become more prevalent.

“Scenarios project a combination of less early winter snowfall and earlier snowmelt, leading to a shorter snow season,” the report says.

The report also calls for a greater percentage of winter precipitation falling as rain, a northward shift in the rain-snow line and fewer days with snow on the ground.

This is all consistent with what Washington is already observing.

Over time, Washington has seen the number of accumulating snow events per winter decrease from an average of six to three. The sharpest decreases in snow have occurred on the bookends of winter, in November and December and March and April, as warming temperatures have flipped what might have fallen as snow to rain. All five of D.C.’s latest measurable snows on record (in April) occurred before 1960.

High-tide flooding ‘every other day’ later this century in coastal areas

Sea levels in the Mid-Atlantic have risen “three to four times” as fast as the global average rate in recent decades, contributing to a “100 to 200 percent increase in high tide flooding in some places,” the report says.

Seas rose about a foot in the last century, and projections are for another 2 to 4.5 feet of rise in the Mid-Atlantic in the next 100 years. A recent NOAA study, which the report cites, projects that these rises in sea level will result in high-tide flooding “every other day” (182 days per year) or more by 2100.

This high-tide flooding will manifest itself even on sunny days but will be most severe during storms that push added water into coastal zones.

“Sea level rise of two feet, without any changes in storms, would more than triple the frequency of dangerous coastal flooding,” the report says.

Closing thoughts

Later this century, the projections laid out above make clear that the weather we experience and the condition of our coasts will fundamentally differ from today. How radically different the planet becomes will depend on whether greenhouse gas emissions slow or continue on a business-as-usual path.

But even if we slow emissions, a substantial amount of warming is already cooked into the system, so certain effects are unavoidable. How disruptive they become to our way of life and how costly will depend on how attentively and successfully we adapt to and plan for them.

How will we protect vulnerable groups like the homeless from suffocating summer heat? Will our power companies have sufficient energy to supply air-conditioning during heat waves? What will the longer, hotter warm season mean for farmers and the kinds of crops they grow? How will we hold back the water and harden our infrastructure in flood-prone zones such as Old Town, Annapolis, Georgetown and Ellicott City? How will Mid-Atlantic ski resorts, which already are challenged to keep terrain open given frequent winter thaws, withstand more-frequent and more-intense warm spells?

The challenges posed by climate change to our region’s infrastructure, energy supply, agriculture and forests, our health, and the environment are layered and complex and go well beyond the weather forecast. They will test our society’s resilience and ability to achieve a sustainable future.

To better understand the implications of climate change on all sectors of society and possible adaptation strategies, the individual chapters of this report are well worth reading and merit consideration for anyone concerned about the long-term health of our local region and its ability to thrive.

President Trump on Nov. 26 reacted to a major report issued Nov. 23 that said climate change will challenge the economy, environment, and human health. (The Washington Post)