During the past few months, Scott Pruitt, Donald Trump’s pick to head the Environmental Protection Agency, Rick Perry, Trump’s pick to head the Energy Department, and Donald Trump himself have all said that they don’t believe in climate change. The most upsetting part of their statements has been the misuse of the word believe.

Religion is a belief system. You have to believe that Moses parted the Red Sea or that Lazarus was raised from the dead. Because these phenomena violate the laws of nature, they are matters of faith.

Science, on the other hand, isn’t a belief system. It’s an evidence-based system. For example, you don’t have to believe in the theory of evolution. All you have to do is examine 250,000 years of fossil records to know that humans and apes evolved from a common ancestor. You don’t have to believe in the germ theory. All you have to do is recognize that vaccines, antibiotics, and sanitation programs have increased our lifespan by 30 years during the past century. And you don’t have to believe in the theory of gravity. All you have to do is drop your pen. None of these concepts are theories any more. They’re facts supported by evidence.

So what about climate change? Where is the evidence? To know whether climate change exists, you need to answer only one question: Have human activities increased the amount of carbon dioxide (CO 2 ) in the environment? The clearest answer to this question can be found in Michael Mann’s, The Madhouse Effect: How Climate Change Denial Is Threatening Our Planet, Destroying Our Politics, and Driving Us Crazy (Columbia University Press, 2016). Mann, a Distinguished Professor of Atmospheric Science at Penn State University, is a leader in the field.

CO 2 is released into the atmosphere by the burning of fossil fuels such as coal, oil, and natural gas, all of which are derived from the remains of living organisms (hence the word fossil). Before the Industrial Revolution, scientists detected about 280 parts of CO 2 per million parts of atmosphere (parts per million or ppm). Today, that figure has risen to about 400 ppm. These studies, however, only looked back over the last 200 years or so. To look further back in time, scientists have examined ice cores generated in places like the North Pole, which allowed them to determine CO 2 concentrations thousands of years ago. Examination of these cores showed that we have now reached CO 2 levels in the atmosphere that haven’t been seen since the dawn of human civilization (about 10,000 years ago) and probably as long as 5 million years ago.

The question at this point is who cares. Why does it matter that CO 2 levels in the atmosphere have increased? For this answer we turn to Joseph Fourier, a French mathematician and physicist, who made the single most important observation in all of climate science: CO 2 is a heat-trapping gas. As levels of CO 2 in the atmosphere increase, the temperature on the surface of the Earth will also increase. This is called “the greenhouse effect.”

Sometimes people like Donald Trump, Scott Pruitt, and Rick Perry describe climate change as a “controversial new science.” In truth, it’s not new at all. Joseph Fourier made his observations in the early 1800s, well before Charles Darwin advanced his theory of evolution.

Because of rising CO 2 levels, the Earth’s surface temperature has increased 1.5oF (1oC). This might not sound like a lot, but it’s already having an effect. The most obvious outcome has been a series of devastating heat waves, like those that recently affected Europe, India, and Pakistan. In the United States, record daily high temperatures have doubled during the past 50 years. Indeed, California is experiencing its worst drought in more than a millennium.

Increased temperatures have caused a melting of the Greenland and West Antarctic Ice Sheets, resulting in a rise in ocean levels of about 10 inches; by the end of the century it will be 3 feet. This 10-inch rise in ocean levels has already caused problems for low-lying island nations like those in the tropical Pacific. Rising oceans also provided for an ironic moment during the 2012 presidential race when Republican presidential hopeful Mitt Romney mocked President Barack Obama for expressing his concern. A few weeks later, Superstorm Sandy hit the coast of New Jersey. The global sea level rise added another 25 square miles and $2 billion to the devastation, according to Mann. Storms like Superstorm Sandy, once expected to occur every century, are now expected to occur every few years.

Increasing numbers of heat waves, melting ice caps, and rising oceans aren’t the only problems caused by global warming. Warmer air holds more moisture than cooler air, producing more rain, snow, and flooding. That’s why it was possible for Texas (the home of Rick Perry) to experience both a record drought in the summer of 2011 and a record flood in the spring of 2015. Pestilence will also have its day. As the Earth’s surface warms, mosquito-borne illnesses like dengue, malaria, and Zika will spread beyond tropical regions.

At present, about 30 billion tons of CO 2 are released into the atmosphere every year. At this rate, CO 2 concentrations should reach 550 ppm by midcentury. At that point, the earth’s temperature will have risen about 5.5oF (3oC), beyond the tipping point, when it will be too late to reverse the trend. Typically, geologists divide the Earth’s history into a series of eras, like Mesozoic or Paleozoic. The current era is called the Anthropocene, meaning that the biggest threat to the planet isn’t meteors or tectonic shifts in land masses; it’s human activity.

It’s not too late. By turning away from fossil fuels and turning toward renewable sources of energy like those generated by the sun, wind, and rivers, there’s still plenty of time to reverse this trend. The Paris climate summit in 2015 and President Obama’s Clean Power Plan in the same year were good starts. And while it is reasonable for politicians to argue whether the programs that have been put in place have been the most efficient way to spend limited resources, it is madness to claim that the problem doesn’t exist.

The simple truth is that it will cost less money to curb CO 2 emissions today than to mitigate the problems created by the greenhouse effect tomorrow. Our politicians owe us that. If they care at all about their legacy, they need to stop denying what is plainly in front of them. Because, as noted by Raymond Aron, “The judgment of history is without pity.”

Paul A. Offit, MD is a professor pediatrics and director of the Vaccine Education Center at the Children’s Hospital of Philadelphia. He is the author of Pandora’s Lab: Seven Stories of Science Gone Wrong (National Geographic Press, April 2017).