Atmospheric carbon dioxide (CO2) has reached yet another milestone, due mainly to the burning of fossil fuels and the destruction of forests.

This May, CO2 has reached 421 parts per million (ppm), meaning that for every million molecules in the air, 421 are xarbon dioxide.

This probably sounds like a miniscule amount, but since carbon dioxide is a heat trapping greenhouse gas, it has a huge effect. Besides being a key ingredient in food for plants, CO2 is necessary for life as we know it.

Combined with water vapor and methane, CO2’s greenhouse abilities help sustain planet Earth at a comfortable 60 degrees Fahrenheit. If somehow all CO2 vanished overnight, Earth’s temperature would fall to near zero in a couple of decades.

But too much of a good thing is not good. Since the dawn of the Industrial Revolution, CO2 has increased from 280 ppm to 421 ppm — a 50% increase. Over the past 800,000 years, CO2 has rarely even exceeded 300 ppm. In fact, carbon dioxide is higher now than it has been in over 4 million years.

This has increased the amount of heat being stored in the Earth’s system, and increased global temperatures by about 2 degrees Fahrenheit.

In the below graphic, you can see just how abnormal this abrupt increase in heat has been over the past 2000 years, with blue bars representing temperatures below the modern day normal and red showing above normal temperatures.

Credit: Warming Stripes from Ed Hawkins

Two degrees may not sound like a lot, but if your body temperature spikes from 99 to 101, you’ll certainly feel under the weather.

At the current rate of increase in greenhouse gases like CO2, the planet is headed for at least another 2 to 3 degrees Fahrenheit of warming this century. The Earth’s fever would then be comparable to a human fever of 104. This is the kind of Earth fever in which interconnected support systems and ecosystem services break down.

The understanding that carbon dioxide has been warming the Earth is not new science. It’s been well understood since the 1960s. But despite this knowledge, humanity has not slowed the burning of fossil fuels, and as a result, the trapping of heat in the climate system continues to rise.

A NOAA study released just days ago shows that greenhouse gas pollution traps 49% more heat today than it did just 30 years ago, in 1990.

Although not an easy task, if humans were able to quickly stop releasing excess carbon pollution, Earth’s temperature would stop warming almost immediately, although many of the impacts would linger for decades and centuries, like sea level rise.

The below image shows the choice humanity has before it. On the left, the map shows the temperature increase projected if we significantly reduce the burning of fossil fuels and thus emissions soon. On the right, the map shows what happens of humans continue to emit greenhouse gases at the same rate as today.

What we decide may mean the difference between inconvenience (left) and catastrophe (right).