About the authors
Michael E. Mann is a professor of atmospheric science and director of the Earth System Science Center at Penn State University. He co-authored, with Washington Post cartoonist Tom Toles, “The Madhouse Effect: How Climate Change Denial Is Threatening Our Planet, Destroying Our Politics, and Driving Us Crazy.”
Susan J. Hassol is the director of Climate Communication LLC. She recently authored "(Un)Natural Disasters: Communicating Linkages Between Extreme Events and Climate Change” in the World Meteorological Organization Bulletin.
Thomas C. Peterson is president of the Commission for Climatology of the World Meteorological Organization. He was formerly principal scientist at NOAA’s National Climatic Data Center and National Centers for Environmental Information.
NOAA hurricane hunters flew through the eye of Hurricane Irma on Sept. 5, capturing the storm's massive eye wall on camera. (Reuters)

As we begin to clean up from Hurricane Harvey, the wettest hurricane on record, dumping up to 50 inches of rain on Houston in three days, and await landfall of Irma, the most powerful hurricane on record in the open Atlantic Ocean, people are asking: What is the role of human-induced climate change in these events, and how else have our own actions increased our risks?

Fundamental physical principles and observed weather trends mean we already know some of the answers — and we have for a long time.

Hurricanes get their energy from warm ocean waters, and the oceans are warming because of the human-caused buildup of heat-trapping gases in the atmosphere, primarily from the burning of coal, oil and gas. The strongest hurricanes have gotten stronger because of global warming. Over the past two years, we have witnessed the most intense hurricanes on record for the globe, both hemispheres, the Pacific and now, with Irma, the Atlantic.

We also know that warmer air holds more moisture, and the amount of water vapor in the atmosphere has increased because of human-induced global warming. We’ve measured this increase, and it has been unequivocally attributed to human-caused warming. That extra moisture causes heavier rainfall, which has also been observed and attributed to our influence on climate. We know that rainfall rates in hurricanes are expected to increase in a warmer world, and now we’re living that reality.

And global warming also means higher sea levels, both because ocean water expands as it warms and because ice in the mountains and at the poles melts and makes its way into oceans. Sea level rise is accelerating, and storm surge from hurricanes rides on top of higher seas to infiltrate further into our coastal cities.

Heavier rain and higher sea levels can combine to compound flooding in major hurricanes, as the deluges cause flooding that must drain to the sea but can’t do so as quickly because of storm surges. Sadly, we saw this effect in play in the catastrophic flooding from Harvey.

We don’t have all of the answers yet. There are scientific linkages we’re still trying to work out. Harvey, like Hurricane Irene before it in 2011, resulted in record flooding, because of a combination of factors. Very warm ocean temperatures meant more moisture in the atmosphere to produce heavy rainfall, yes. But both storms were also very slow-moving, nearly stationary at times, which means that rain fell over the same areas for an extended period.

Cutting-edge climate science suggests that such stalled weather patterns could result from a slowed jet stream, itself a consequence — through principles of atmospheric science — of the accelerated warming of the Arctic. This is a reminder of how climate changes in far-off regions such as the North Pole can have very real effects on extreme weather faced here in the Lower 48.

These linkages are preliminary, and scientists are still actively studying them. But they are a reminder that surprises may be in store — and not welcome ones — when it comes to the unfolding effects of climate change.

Which leads us, inevitably, to a discussion of policy — and, indeed, politics. Previous administrations focused on adapting to climate change, with an eye to what the planet would look like in the future. But events such as Harvey, and probably Irma, show that we have not even adapted to our current climate (which has already changed because of our influence).

The effects of climate change are no longer subtle. We are seeing them play out before us here and now. And they will only worsen if we fail to act.

The Trump administration, however, seems determined to lead us backward. In recent months, we have witnessed a dismantling of the policies put in place by the Obama administration to (a) incentivize the necessary move from climate-change-producing fossil fuels toward clean energy, (b) increase resilience to climate change effects through sensible regulations on coastal development, and (c) continue to fund basic climate research that can inform our assessments of risk and adaptive strategies. Ironically, just 10 days before Harvey struck, President Trump rescinded flood protection standards put in place by the Obama administration that would take sea level rise and other climate change effects into account in coastal development plans.

And as Trump kills policies that would reduce the risks of climate disasters, our nation continues to support policies that actually increase our risks. For example, without the taxpayer-subsidized National Flood Insurance Program, banks would be less likely to provide mortgages for rebuilding houses in locations that have been flooded before, sometimes repeatedly. And the flood insurance program is itself underwater: badly in debt and set to expire at the end of this month unless Congress finds a way to keep it afloat, just as billions of dollars in claims from Harvey come pouring in.

Harvey and Irma are sad reminders that policy matters. At a time when damage from climate change is escalating, we need sensible policy in Washington to protect the citizens of this country, both by reducing future climate change and preparing for its consequences. We should demand better of our leaders.

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