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Even with the carbon release of mankind so far, there is far less carbon dioxide in the earth’s atmosphere than Venus, and the earth is farther from the sun. But if carbon emissions continue at the current rate, is there a risk of reaching a tipping point at which a runaway culture effect takes over, making the earth uninhabitable for any form of life?
When sunlight enters the earth’s atmosphere, some are reflected back into space by clouds, others are reflected by bright surfaces such as ice and snow, and others are absorbed by the land surface and the ocean.
To maintain a balance, the earth sends energy back to space in the form of infrared or long-wave radiation. Some long-wave radiation is absorbed into the atmosphere by gases such as carbon dioxide.
This is the well-known greenhouse effect.
Read more: Climate explains: what would the earth be like if we did not pump greenhouse gases into the atmosphere?
As already established, the concentrations of carbon dioxide have increased over the past 250 years, causing the average surface temperature to rise.
One consequence of increasing atmospheric carbon dioxide concentrations is that as the atmosphere warms up, it may contain more water vapor. Since water vapor itself is a greenhouse gas, it can create a strengthening effect.
In general, as the surface temperature rises, the earth sends more long-wave radiation to space to maintain its energy balance. But there is a limit to how much long-wave radiation can be emitted.
If the atmosphere is completely saturated with water vapor, the earth’s surface and the lower atmosphere are warmed up, but further increase in the emission of longwave radiation is not possible.
The runaway greenhouse
It is called a runaway greenhouse and would mean that the earth would become deadly hot and could not cool itself by giving off heat to space.
Ultimately, it is the fate of the earth. In billions of years from now, the sun will be brighter and grow into a red dwarf. As the sun’s brightness increases, the earth will become warmer and its oceans will evaporate.
The warm and steamy atmosphere will ensure that the earth is just as uninhabitable for current life forms as Venus is today.
But can we achieve such a situation in a shorter period of time through continued carbon dioxide emissions? 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 probably insufficient to cause a runaway greenhouse.
Atmospheric carbon dioxide is currently about 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 occurs over a short period of time. Yet human emissions of carbon dioxide are considered insufficient to cause a runaway greenhouse, given the available fossil fuel reserves.
The earth must be safe from a runaway greenhouse for at least another 1.5 billion years.
But then…
The caveat to all of the above is that the models that scientists use to study future climates are based on known, known conditions. It is therefore difficult to predict how certain parts of the climate system may operate under extremely high greenhouse gas emission scenarios.
Flickr / scheendijk, CC BY
Clouds, for example, can reflect sunlight back into space, or they can capture heat radiated by the earth. In a warming world, scientists are still unclear about the role that clouds will play.
Read more: Expect the new normal for NZ’s temperature to warm up
Although a runaway greenhouse would make the earth, as we know it, completely uninhabitable, the losses that can occur due to a few degrees Celsius to global warming are serious and cannot be reduced.
Sea level rise, increased frequency and intensity of extreme weather conditions, threats to endangered species and unique ecosystems are just some of the many reasons why we should be concerned.
The silver lining is that we (probably) do not have to worry about becoming like our neighbor Venus anytime soon.