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Even with all of humanity’s carbon emissions to date, there is much less carbon dioxide in Earth’s atmosphere than Venus, and Earth is farther from the Sun. But if carbon emissions continue at the current rate, will Is there any risk of reaching a tipping point where a runaway greenhouse effect takes over, rendering the Earth uninhabitable for any form of life?
When sunlight enters Earth’s atmosphere, some is reflected back into space by clouds, some is reflected off shiny surfaces like ice and snow, and some is absorbed by the earth’s surface and the ocean.
To maintain balance, the Earth emits energy back into space in the form of infrared or long-wave radiation. Some of the long-wave radiation is absorbed in the atmosphere by heat-trapping gases, such as carbon dioxide.
This is the well-known greenhouse effect.
Read more: Climate explanation: what the Earth would be like if we had not pumped greenhouse gases into the atmosphere
As is well established, carbon dioxide concentrations have increased over the past 250 years, causing the average surface temperature to rise.
One consequence of increasing carbon dioxide concentrations in the atmosphere is that, as the atmosphere warms, it can hold more water vapor. Since water vapor is itself a greenhouse gas, this can create an amplifying effect.
In general, as the surface temperature increases, the Earth emits more long-wave radiation into space to maintain energy balance. But there is a limit to the amount of long-wave radiation that can be emitted.
If the atmosphere is completely saturated with water vapor, the Earth’s surface and the lower atmosphere become warm, but the emission of long-wave radiation cannot be further increased.
The unbridled greenhouse
This is called a runaway greenhouse and would mean that the Earth would become lethally hot and could not cool itself by emitting heat into space.
Ultimately, this is the fate of the Earth. Billions of years from now, the Sun will get brighter and turn into a red dwarf. As the Sun’s luminosity increases, the Earth will heat up and its oceans will evaporate.
The warm, smoky atmosphere will ensure that Earth is as uninhabitable for current life forms as Venus is today.
But could we provoke such a situation in a shorter period of time through continuous emissions of carbon dioxide? The good news is, probably not.
We are safe for now
Previous research has found that due to differences in the properties of water vapor and carbon dioxide as greenhouse gases, the addition of carbon dioxide to the atmosphere is likely insufficient to cause a runaway greenhouse effect.
Atmospheric carbon dioxide is currently around 416 parts per million (ppm), up from about 280 ppm since the first industrial revolution began, about 250 years ago.
In geological terms, this is a very large increase that will occur in a short period of time. However, human emissions of carbon dioxide are considered insufficient to cause a runaway greenhouse effect, given the available fossil fuel reserves.
Earth should be safe from a runaway greenhouse that has been developing for at least another 1.5 billion years.
But then …
The caveat to all of the above is that the models scientists use to study future climate are built on known past conditions. Therefore, it is difficult to predict how certain parts of the climate system might operate under extremely high greenhouse gas emissions scenarios.
For example, clouds can reflect sunlight back into space or they can trap heat emitted by Earth. In a warming world, scientists are still unclear about the role that clouds will play.
Read more: Wait for the new normal for the NZ temperature to warm up
While a runaway greenhouse would make the Earth completely uninhabitable for life as we know it, the losses that can stem from a few degrees Celsius of global warming are serious and should not be ruled out.
Rising sea levels, increasing frequency and intensity of extreme weather events, threats to endangered species, and unique ecosystems are just a few of the many reasons we need to be concerned.
The silver lining is that we (probably) don’t have to worry about becoming like our neighbor Venus anytime soon.