President Trump announced last week that he will be pulling the United States out of the Paris Climate Accord, the world’s best shot at keeping climate change to survivable levels. It is a short-sighted decision that is going to put US companies at global competitive disadvantage.

It’s also going to harm human health, because climate change is our species’ biggest health threat. When we think about climate change, we usually think of environmental impacts and damage to the planet. The health consequences of climate change, however, will be massive.

If the Paris Accord falls apart and climate change goes unchecked, this decision will kill people.

Climate change and human health are tied, in thousands and thousands of direct and indirect ways.

There are direct impacts like extreme weather, heat waves, and rising sea levels. Massive storms, and more frequent storms, will kill people. People are killed by hurricanes, for example, by the initial storm damage and from the infrastructure damage that results. Over the last three decades, hurricanes have gotten weird: Fewer hurricanes occur, but the ones that do occur are very strong – stronger than the old average. The strongest hurricanes have increased in intensity; really big storms are getting even bigger.

The science of hurricane modeling is extremely complex, but big storms are much deadlier than small storms. Fewer, bigger storms, is not an improvement. 1,000 people died in Haiti as a result of Hurricane Matthew’s massive impact. Future giant hurricanes will have the same deadly effect on coastal areas. We have less data on blizzards, tornadoes, and other kinds of storms, but they’re also deadly and getting deadlier.

Another example: people also die from heat alone. In 2003, 14,802 people died of overheating (hyperthermia) in France during a major heat wave. Research has shown that the risk of death from heat in Paris will increase 70% by 2030, and in London by 20%. This isn’t limited to Europe where people aren’t accustomed to heat; 2500 people died in a heatwave in India in 2015. The risk of death from heat is especially severe when there is limited access to clean drinking water. Waterborne diseases prefer warmer water, so climate change will both make clean water more important for health and make that clean water less available.

Climate change also has subtler follow-on impacts.

It will lead to an increase in vector-borne diseases like malaria, Zika, and dengue fever, as mosquitoes and other tropical disease vectors expand their territory. Dengue has actually re-emerged in countries where it had once been eliminated, including the US, supported by warmer weather and higher humidity. Eastern Brazil, the eastern United States, Western and Central Europe, and Eastern China are especially likely to see an increase in disease vectors like the Asian Tiger mosquito, the result of warmer winter temperatures.

Those are just examples. They’re not the whole impact of climate change. There is also the deadly impacts of destroyed infrastructure from rising sea levels, reduced agricultural production and less nutritious food, or wildfires, among other effects. The impacts of climate change on human health are in fact so complicated and varied in type we can’t conveniently quantify them. An estimate published in The Lancet found that half a million people will die every year by 2030 as a result of climate change’s impact on agriculture alone.

We just don’t know how all the different effects will work when taken together. All we know is that the impact will be massive and complicated – and it has already begun. A UN study found that 400,000 people a year are already dying as a result of climate change.

That makes climate change orders of magnitude more deadly than terrorism.

Climate change isn’t about saving the polar bears, or the icecaps, or protecting biodiversity – important as they all are. It’s about people. The children being born today with a whole lifetime of impact to face, the elderly people who will suffer most from disease and injury, and all of us living through it. Exiting the Paris Accord – deliberately allowing climate change to accelerate – is a shameful act of cruelty to the entire human race.