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Even with all of humanity’s carbon emissions to date, there is much less carbon dioxide in the Earth’s atmosphere than Venus, and the Earth is further away from the sun. But if carbon emissions continue at the current rate, is there any risk of reaching a turning point where an ongoing greenhouse effect takes over, making the Earth uninhabitable for any kind of life?
When sunlight enters the Earth’s atmosphere, something is reflected back into space by clouds, something is reflected by bright surfaces such as ice and snow, and something is absorbed by the land surface and the ocean.
To maintain a balance, the Earth emits energy back into space in the form of infrared or long-wave radiation. Some long-wave radiation is absorbed into the atmosphere by heat-trapping gases, such as carbon dioxide.
This is the well-known greenhouse effect.
Read more: Climate explained: what the Earth would be like if we had not pumped greenhouse gases into the atmosphere
As is already well established, carbon dioxide concentrations have risen over the last 250 years, causing the average surface temperature to rise.
One consequence of increasing atmospheric carbon dioxide concentrations is that when the atmosphere is heated, it may contain more water vapor. Since water vapor is itself a greenhouse gas, this can create an amplifying effect.
In general, when surface temperature rises, the Earth emits more long-wave radiation into space to maintain energy balance. But there is a limit to how much long-wave radiation can be emitted.
If the atmosphere becomes completely saturated with water vapor, the earth’s surface and lower atmosphere are heated, but further increases in the emission of long-wave radiation are not possible.
The running greenhouse
This is called a running greenhouse and would mean that the Earth would become deadly hot and unable to cool itself by emitting heat to space.
Ultimately, this is the fate of the Earth. In billions of years, the Sun will be brighter and grow into a red dwarf. As the brightness of the sun increases, the earth will become warmer and its oceans will evaporate.
The warm and steamy atmosphere will ensure that the Earth is as uninhabitable for current life forms as Venus is today.
But could we create such a situation in a shorter time frame through continued carbon dioxide emissions? The good news is probably not.
We are safe so far
Previous studies have shown 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 probably insufficient to trigger a continuous greenhouse.
Atmospheric carbon dioxide is currently around 416 parts per year. Million (ppm) – an increase from approx. 280 ppm since the first industrial revolution began about 250 years ago.
In geological terms, this is a very large increase that takes place over a short period of time. Yet human emissions of carbon dioxide are considered insufficient to trigger an ongoing greenhouse given the available fossil fuel reserves.
The earth must be safe from a continuous greenhouse that develops for at least another 1.5 billion years.
The caveat to all of the above is that the models that scientists use to study future climates are based on previously known conditions. It is therefore difficult to predict how certain parts of the climate system can function under extremely high greenhouse gas emission scenarios.
For example, clouds may reflect sunlight back into space, or they may capture heat emitted by the earth. In a warming world, scientists are still unclear about what role the clouds will play.
Read more: Expect the new normal for NZ’s temperature to get warmer
While a running greenhouse would make the Earth completely uninhabitable for life as we know it, the losses that can occur from a few degrees Celsius of global warming are serious and must not be discounted.
Sea level rise, increased frequency and intensity of extreme weather events, threats to endangered species and unique ecosystems are just a few of the many reasons why we are concerned.
The silver lining is that we (probably) do not have to worry about becoming like our neighbor Venus soon.