Climate explained: rising carbon emissions (probably) won’t make the Earth uninhabitable

Shutterstock/Jurik Peter

Laura Revell, University of Canterbury


Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.

If you have a question you’d like an expert to answer, please send it to

Even with all humanity’s carbon emissions to date, there’s a lot less carbon dioxide in Earth’s atmosphere than Venus, and Earth is further away from the Sun. But if carbon emissions continue at the current rate, is there any risk of reaching a tipping point at which a runaway greenhouse effect takes over, making Earth uninhabitable for any form of life?

When sunlight enters the Earth’s atmosphere, some is reflected back to space by clouds, some is reflected by bright surfaces such as ice and snow and some is absorbed by the land surface and ocean.

To maintain a balance, the Earth emits energy back to space in the form of infrared, or longwave, radiation. Some longwave radiation is absorbed in the atmosphere by heat-trapping gases, such as carbon dioxide.

This is the well-known greenhouse effect.

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Climate Explained: what Earth would be like if we hadn’t pumped greenhouse gases into the atmosphere

As is already well established, concentrations of carbon dioxide have increased over the past 250 years, causing the average surface temperature to increase.

One consequence of increasing atmospheric carbon dioxide concentrations is that, as the atmosphere warms, it can contain more water vapour. Since water vapour is itself a greenhouse gas, this can create an amplifying effect.

In general, as surface temperature increases, the Earth emits more longwave radiation to space to maintain the energy balance. But there is a limit to how much longwave radiation can be emitted.

If the atmosphere becomes completely saturated with water vapour, the Earth’s surface and lower atmosphere warm up, but further increases in emission of longwave radiation are not possible.

The runaway greenhouse

This is termed a runaway greenhouse and would mean the Earth would become lethally hot and unable to cool itself by emitting heat to space.

Ultimately, this is the fate of the Earth. In billions of years from now the Sun will become brighter and grow into a Red Dwarf. As the Sun’s luminosity increases, the Earth will become hotter and its oceans will evaporate.

We’re doomed … but not for billions of years.

The hot and steamy atmosphere will ensure the Earth is just as uninhabitable to current life-forms as Venus is today.

But could we bring such a situation about on a shorter timeframe through continued carbon dioxide emissions? The good news is, probably not.

We’re safe, for now

Previous research has found that, due to differences in the properties of water vapour and carbon dioxide as greenhouse gases, adding carbon dioxide to the atmosphere is likely insufficient to trigger a runaway greenhouse.

Atmospheric carbon dioxide is currently around 416 parts per million (ppm) – up from approximately 280 ppm since the first industrial revolution began, some 250 years ago.

In geological terms, this is a very large increase to take place over a short period of time. Yet human emissions of carbon dioxide are considered insufficient to trigger a runaway greenhouse, given the fossil fuel reserves available.

The Earth should be safe from a runaway greenhouse developing for at least another 1.5 billion years.

But then …

The caveat to all the above is that the models scientists use to study future climate are built based on past, known conditions. It is therefore difficult to predict how certain parts of the climate system might operate under extremely high greenhouse gas emissions scenarios.

Clouds hiding the Sun but with rays of light emerging from behind top.
Clouds can reflect sunlight back to space.
Flickr/scheendijk, CC BY

For example, clouds can reflect sunlight back to space, or they can trap heat emitted by the Earth. In a warming world, scientists are still unclear on the role clouds will play.

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While a runaway greenhouse would make Earth completely uninhabitable to life as we know it, the losses that may accrue from just 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 we have to be concerned.

The silver lining is we (probably) don’t need to worry about becoming like our neighbour Venus any time soon.The Conversation

We’re not heading this way just yet.

Laura Revell, Senior Lecturer in Environmental Physics, University of Canterbury

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Climate explained: will the tropics eventually become uninhabitable?

Flickr/, CC BY-NC-ND

James Shulmeister, University of Canterbury


Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.

If you have a question you’d like an expert to answer, please send it to

What is the impact of temperature increases in the tropics? How likely is it that regions along the Equator will be uninhabitable due to high wet bulb temperatures such as 35℃ and more in places like Singapore? Do we have models that suggest how likely this is and at what time frames?

More than 3.3 billion people live in the tropics, representing about 40% of the world’s population. Despite some areas of affluence, such as Singapore, the tropics are also home to about 85% of the world’s poorest people and are therefore particularly susceptible to the impacts of climate change.

The tropics are expected to experience rising temperatures and changes to rainfall, and the question is whether this could make this region uninhabitable. How would this happen?

Heat stress

Humans regulate their body temperature in warm conditions through sweating. The sweat evaporates and cools the skin. But if conditions are humid, sweating and evaporation are much less effective.

Humans can survive and function in quite high temperatures if humidity is low, but as humidity increases our ability to function decreases rapidly. This effect is measured by a heat stress index which shows the apparent temperature you feel under different relative humidity conditions.

From a human health point of view, the wet bulb temperature is critical. This is the temperature a thermometer covered in a wet cloth would measure, and it reflects the maximum amount of cooling that can be achieved by evaporation.

High wet bulb temperatures are more problematic to human health than high absolute temperatures. Wet bulb temperatures above 35℃ are life-threatening because they cause hyperthermia, which means the body cannot cool down and the internal body temperature exceeds 40℃.

Climate modelling predictions used by the Intergovernmental Panel on Climate Change (IPCC) for the period from 2080-2100 suggest warming in the tropics of about 1.6℃ under mid-range emissions scenarios and up to 3.3℃ under high emissions scenarios, with error margins of about 0.5℃ on both predictions.

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Different parts of the world respond in different ways to warming from greenhouse gas emissions. The projected warming in the tropics represents about 40% of the expected temperature rise in the Arctic.

High-latitude regions – far north or south of the Equator – warm more rapidly than the global average because excess heat in the tropics creates a temperature and pressure gradient. This drives heat up to higher elevations and higher latitudes through an atmospheric circulation called the Hadley cell.

The stronger the gradient, the more heat is exported.

Hot in the city

There is one additional factor: urbanisation. Singapore is a good place to look at actual climate change in the tropics.

A Singapore skyline with clouds and some sun breaking through.
Cities such as Singapore will get hotter.
Flickr/Mohammad Hasan, CC BY-NC

Records from Singapore indicate temperatures have increased by 1.1℃ over 42 years to 2014. This is nearly twice the average global rate of warming over recent decades and is opposite to expectations.

The difference appears to be due to a heat island effect caused by the city itself. This is important because changes in land use amplify background global climate change and put tropical cities at greater risk of extreme heat. As populations are concentrated in cities, this increases the risk to human health.

The mean average temperature for Singapore is about 27℃, whereas Jakarta in Indonesia is slightly warmer. At the scale of predicted mean annual temperature change, neither of these cities would become uninhabitable. But even a small temperature increase would make life more challenging.

This is made worse in at least some parts of the tropics, because total rainfall is increasing, suggesting a long-term rise in humidity. For example, average rainfall in Singapore increased by more than 500mm from 2,192mm in 1980 to 2,727mm in 2014.

Read more:
Urban growth, heat islands, humidity, climate change: the costs multiply in tropical cities

Deadly heat

People working outdoors are at higher risk, as are vulnerable populations, including the elderly. Under the IPCC’s high-emission trajectory, heat-related deaths in Jakarta in August are expected to rise from about 1,800 in 2010 to nearly 27,000 in 2050.

People unloading cargo in the outdoors at Jakarta port.
Working outdoors in the increased heat and humidity will get harder.
Flickr/Jorien, CC BY-NC

Even allowing for a significant increase in elderly people as the Indonesian population ages, this means about 15,000 excess deaths in this month. Estimates under high-emission predictions for the tropics and mid-latitudes suggest about a 40% decline in the ability to undertake manual work during the warmest month by 2050.

These impacts will be stronger in the seasonally wet tropics (such as the Northern Territory of Australia), where more extreme warming is expected than in the equatorial zone.

Predictions for Darwin, in northern Australia, suggest an increase in days with temperatures above 35℃ from 11 days a year in 2015 to an average of 43 days under the mid-range emission scenario (IPCC’s RCP4.5 scenario) by 2030 and an average of 111 (range 54-211) days by 2090. Under the higher emission scenario (IPCC’s RCP8.5), an average of 265 days above 35℃ could be reached by 2090.

In summary, while absolute temperatures are expected to rise more slowly in the tropics when compared with higher latitudes and polar regions, the combination of heat and rising humidity will make life challenging, but not impossible.The Conversation

James Shulmeister, Professor, School of Earth and Environmental Sciences, University of Canterbury

This article is republished from The Conversation under a Creative Commons license. Read the original article.