For decades, the world has been told that the climate is changing—that the build-up of fossil fuel-driven greenhouse gas emissions would irrevocably change the Earth's systems.
Those changes are already happening across the United States, the newest volume of the National Climate Assessment says. The exhaustive report, written by scientists and released Friday by 13 federal agencies (and, to the surprise of some, signed off on by the Trump Administration), also clearly states that humans have directly contributed to the warming of the globe.
"This period is now the warmest in the history of modern civilization," the report's authors wrote.
Globally, surface air temperatures have increased by roughly 1.8 degrees Fahrenheit over the last 115 years.
That warmth doesn't happen in a vacuum, thanks to the planet's complicated, interlocked systems. As temperatures rise, sea ice melts and permafrost thaws, the oceans warm and fuel stronger storms, and forests dry out, fueling destructive fires. The ripple may seem small, but its cumulative impact, the report warns, could look more like a tsunami.
Sea level is rising, and extreme weather events that used to be anomalies are becoming the norm.
Each chapter of the nearly 500-page report deals with a different element of climate change, from the present-day impacts to projections for the future. Those projections vary greatly based on which pathway the world takes—one that shifts and limits carbon emissions, or one that continues to burn fossil fuels without hesitation. But what's clear, too, is that the changes have already begun.
It's Getting Hot in Here
Recent decades in the U.S. have been the hottest in the past 1,500 years, the report says.
Across the country, temperatures have increased by an average of about 1.23°F, but the impacts vary by region.
In Alaska, the average temperature has climbed roughly 1.7°F, and extreme spikes in temperature are happening more frequently. In Deadhorse, Alaska—home to the airport that services much of the oil and gas drilling in Prudhoe Bay—the temperature hit 85°F on July 13, 2016, setting a record for hottest temperature within 50 miles of the Arctic Ocean in Alaska.
What's to come? Though climate scientists warn that temperatures must be kept within 3.6°F (2°C) warming above preindustrial temperatures to avoid the worst consequences of climate change, that goal is seeming harder to achieve.
The climate assessment warns that without major reductions in emissions, global temperatures could hit 9°F (5°C) or more by the end of this century.
Extreme Weather, Drought and Wildfires
Extreme weather events like we've seen this hurricane season are on the rise. Scientists project that while the number of hurricanes isn't likely to increase, their intensity is, and thanks to important advances in the field of attribution studies, scientists are now able to look at individual events and find the fingerprints of climate change.
Heavy precipitation events across the country have increased in both intensity and frequency since 1901, according to the assessment. Organized clusters of thunderstorms—called mesoscale convective systems—have been happening more frequently and with greater precipitation amounts since 1979.
Annual rainfall has been on the rise, mostly in the Northern and Southern Plains, Midwest and Northeast. But other areas, like the West, Southwest and Southeast, have seen a decline.
Since 2011, the United States has experienced a number of "very significant droughts," with varying degrees of human attribution, the assessment says.
Drier conditions can increase wildfire risk, as parts of the country have seen in recent years. Increases in both temperature and drought in the western U.S., connected to human-caused greenhouse gas emissions, have been found responsible for doubling the region's forest fire area over the period 1984-2015, the assessment says.
Arctic Sea Ice Is Disappearing
Nowhere are the current impacts of climate change more clear than in Alaska. Like the rest of the Arctic, temperatures are rising there nearly twice as fast as the rest of the globe.
In the coldest months, much of the Arctic Ocean is covered in ice—with older, thick ice in some places, and newer, thinner ice that melts and re-forms each year in others. As the climate warms, that ice melts earlier and re-forms later, and the older ice is beginning to melt too. As it does, the dark surface of the ocean absorbs more heat from the sun, advancing the cycle of change. Since 1979, the average Arctic sea ice extent has declined by an estimated 3.5 to 4 percent per decade.
In Alaska, warming is especially strong in the fall. In Utqiagvik (formerly Barrow), warming since 1979 exceeds 7°F (3.8°C) in September, 12°F (6.6°C) in October, and 10°F (5.5°C) in November, according to the assessment.
There's a reason why scientists spend so much time studying what is happening in the Arctic. The impact of warming there is being felt globally, as it influences global sea level, ocean salinity, the carbon cycle and potentially atmospheric and oceanic circulation patterns.
Sea Level Is Rising
Ask the residents of Miami, Florida, or Norfolk, Virginia, and they will tell you: Sea level rise is happening now.
Since 1900, global sea levels have risen about 7-8 inches. Three of those inches of rise have occurred since 1993, according to the assessment. The rate we're experiencing now outpaces any sea level rise in at least 2,800 years.
Sea level rise is driven by two main factors: an increase in the volume of seawater as the ocean expands as it warms, and an increase in mass from the melting of mountain glaciers and the ice sheets of Antarctica and Greenland.
Oceans Are Warming
The oceans have absorbed about 93 percent of the extra heat created by global warming since the 1970s. That has tempered the amount of atmospheric warming and has resulted in changes in the ocean itself—from the increase in volume that's contributing to sea level rise, to heat that can fuel hurricanes and affect sea life, to changes in the ocean's circulation.
From 1900 to 2016, U.S. coastal waters have warmed by more than 0.7°F (0.4°C), according to the assessment. In the past 60 years, ocean warming in three regions of the United States— Alaska, the Northeast, and the Southwest—outpaced the rest of the globe. The Northeast is warming particularly fast—since 2004, surface temperatures there have warmed faster than 99 percent of the global ocean.
Across the U.S. coastal waters, the assessment found that the warming detected can be attributed to manmade climate change—and that it's just the beginning. In a future where high emissions continue, the surface temperatures are projected to rise by another 4.9°F by 2100.