In a recent interview on 60 Minutes, President Donald Trump claimed scientists who “believe” that humans are affecting global climate “have a very big political agenda.”

As a climate scientist, I’d like to respond to this claim.

The president is wrong on multiple counts. Human effects on climate are not a belief system. Climate scientists don’t just “believe” that the amount of carbon dioxide in the atmosphere is increasing. We measure CO2 at dozens of remote locations. We monitor CO2 in bubbles of air in ice cores from Greenland and Antarctica. Our measurements show that global atmospheric CO2 levels increased nearly 40 percent since the Industrial Revolution began in the late 18th century.

Climate scientists don’t just “believe” that these measured CO2 increases are primarily caused by fossil fuel burning. Through careful analyses performed at dozens of labs around the world, we know that fossil fuel burning changes the relative abundance of lighter and heavier forms of atmospheric carbon. We measure how these different forms of carbon change over time. The findings are clear: nearly three quarters of the post-18th–century increase in CO2 is from fossil fuel burning.

Atmospheric physicists and chemists have studied how CO2 traps heat for over 150 years. Initial insights came from lab experiments. Later, scientists examined relationships between CO2 and climate in the Venusian and Martian atmospheres, and in nearly a million years of ice core data. More recently, we’ve learned about the heat-trapping properties of CO2 from measurements of the changing balance between incoming sunlight and Earth’s outgoing planetary warmth.

We now know, beyond a shadow of a doubt, that burning fossil fuels increases atmospheric CO2 levels, and that increased CO2 will warm our planet. It’s still unclear exactly how much warming eventually will occur if we burn enough fossil fuels to double pre-industrial levels of atmospheric CO2. But it’s certain that such a large CO2 change ultimately leads to appreciable warming.

Appreciable warming of the Earth is not something we have to “believe in.” It’s already happening. Warming of our planet is hard physical reality. The average temperature of the land and ocean surface increased by at least 1° C since 1880. The temperature of the lower atmosphere—which we’ve monitored from space since 1979—rose by nearly 0.7° C. This measured warming is consistent with independently monitored changes in ocean heat content, snow, Arctic sea ice, land glaciers, the mass balance of major ice sheets, sea level, atmospheric moisture, salinity and many other climate properties. Thousands of scientists have made these measurements, at hundreds of different research labs and in countries around the world.

Could all of this evidence point towards a natural cause for the observed warming? President Trump seems to think so. His remarks on 60 Minutes imply that Earth is simply in a natural warming phase, and the climate system “could very well go back” to normal conditions.

It would be very comforting if the president were right. Unfortunately, he is not. Natural cycles are an intrinsic part of the climate system and have been studied for decades. They cannot account for simultaneous warming of all major ocean basins. Nor can natural cycles explain the sustained warming of the lower atmosphere and cooling of the upper atmosphere—a characteristic fingerprint of human-caused greenhouse-gas increases. The president’s comment about restorative natural cycles ignores our mature scientific understanding of the strong links between fossil fuel burning, CO2 increase and climate change. Hoping that a small natural cycle will magically cancel out the large climate signal of fossil fuel burning is a very poor survival strategy.

One final word about agendas. Do climate scientists “have a very big political agenda,” as the president has claimed? In my experience, most scientists are focused squarely on getting the science right. At the end of a scientific career, that’s the real measure of an individual’s success: Were your findings credible? Was your research confirmed by others? Did you advance scientific understanding? That’s the “agenda.”

If he so desired, President Trump could go down in history as the man who transcended ego and ignorance; the man who defined his presidency by acting decisively to limit greenhouse gas emissions. His legacy could be positive and enduring. But if he is incapable of understanding the lessons of climate science, he will go down in history as President Ozymandias. His great walls and Trump Towers will be overwhelmed by desert sands and rising seas, forever lost and forgotten.