Climate models and the latest IPCC data reveal four possible futures for global population, economy and environment at the end of this century

How different will it look at the end of this century? (Image: Superstock)

YOU may have heard last week that Earth in 2100 is likely to be between 0.3 and 4.8 °C warmer than it was in the late 20th century. This is one of the key messages of the latest UN Intergovernmental Panel on Climate Change report. Why such a broad range? Because the rise in temperature depends largely on what we choose to do now – where our energy and food come from, the type of buildings we live in and the cars we drive. And of course, how many people populate the planet.

Read more: “IPCC 2013: The latest state of the climate report“


Those choices will determine whether Earth stays much the same as it is today, or if rising seas eat up major land masses – Florida, Bangladesh, we’re looking at you – and large parts of the world become too dry to live in (see diagram).

Climate scientists combine the various options into descriptions of how human society might evolve, and plug them into their models to see what each scenario would mean for the climate. For ease of comparison, they have agreed on two extreme scenarios and a couple in between. That is what determines the 0.3 to 4.8 °C range.

Here, New Scientist explores four hypothetical futures for human society in 2100, using criteria set out by climate modellers – though we cannot reproduce the huge amount of data in their scenarios (see graph). We have selected some key points and sketched out an image of society in each scenario. To do this, we drew on descriptions published by the IPCC in 2000 and, in consultation with climate modellers, chose the ones that correspond to the concentrations of greenhouse gases published in last week’s IPCC report (see “Climate report: Lull in warming doesn’t mean we’re safe“).

It is important to stress that these are just four of the many possible scenarios and ways to cut emissions. But we feel the value of the exercise is in showing how much of the outcome is still in our hands and down to the choices and trade-offs we make as individuals and as society as a whole. For example, a low population opens some options. A high one closes others down, but you could compensate for that by transforming the transport sector, for instance.

Editorial: “The future of the climate is still in our hands“

“We want people to think about what world they want to live in, and then think about ways that they can make that consistent with a climate future that they want,” says Richard Moss of the Pacific Northwest National Laboratory in Richland, Washington, who has been closely involved in developing the climate scenarios.

We want people to think about what sort of world they want to live in

1: Geoengineered safety Population 9 billion

Global energy use 8 × 1020 joules

CO 2 concentration 400 ppm, dropping We acted early in the 21st century, invested aggressively in renewable energies and crucially, geoengineering It wasn’t easy, but by investing heavily in R&D, we have built systems for sucking carbon dioxide out of the atmosphere and storing it underground. At the same time, we have invested in renewable energies and virtually weaned ourselves off fossil fuels. The net result: annual carbon emissions have plummeted, and atmospheric concentrations of CO 2 are finally dropping. Crucial to making this happen was the spread of bioenergy power plants coupled to carbon storage facilities – a soft form of geoengineering. We grew trees and plants to burn and produce electricity. They suck CO 2 out of the atmosphere as they grow. We captured the greenhouse gases produced when we burn trees, and put the gases in geological seams deep underground – where they will stay for centuries or more. Global temperatures have held steady since 2050. We’ve also halted the decline of sea ice in the Arctic and slowed ocean acidification. Sea levels are still rising, though, because of heat stored in the system from earlier emissions.

2: Slight delay Population 8.5 billion

Global energy use 1 × 1021 joules

CO 2 concentration stable at 550 ppm We delayed both the transition to renewable energies, and the implementation of climate treaties As a whole, we are a more efficient society than in 2013, using less energy and fewer materials to produce more. We are also good recyclers. All of this makes for better management of natural resources. Earlier on in the century, gas was the transition fuel of choice. Now most of our energy comes from renewables and nuclear. Incentives and better international institutions mean green technologies spread rapidly. We have made the transition to a low-carbon economy. We eat far less meat than in 2013, to cut the emissions generated by livestock farming. So pastures have shrunk and agriculture is more efficient. Forests are growing, boosting the amount of CO 2 stored in trees around the world. We live in compact cities with excellent public transport which also limits emissions. Although temperatures and sea levels have risen, some of the more extreme consequences of climate change haven’t come to pass.

3: Too little, too late Population 9.5 billion

Global energy use 8 × 1020 joules

CO 2 concentration 650 ppm and rising We cut emissions, but not until late in the century The first half of the century was spent mostly carrying on business as usual: we relied heavily on fossil fuels. We did not introduce any dramatic changes to our life styles or activities in terms of consumption, travel and the number of children we have. Then, towards the middle of the century, the consequences of climate change became too difficult to ignore. As a result, our governments slowly began introducing some unambitious policies to regulate emissions. We are now slowly making our way towards a green energy supply. Oil consumption started to drop a few decades ago, but 75 per cent of our energy still comes from fossil fuels, not much less than the 82 per cent in 2011. Because of our inaction, temperatures are still rising, as are sea levels. Models suggest they will continue doing so for several decades more.

4: Addicted to carbon Population 12.5 billion

Global energy use 1.75 × 1021 joules

CO 2 concentration 950 ppm and rising The world economy is booming, but it’s fuelled by coal and oil. The global population continued to grow rapidly throughout the century, pushing emissions ever higher Welcome to the globalised, high-tech, consumerist future: one where we are still hooked on fossil fuels. Emissions have gone through the roof, and human health and the environment are the casualties. Biodiversity crashes are threatening the normal operation of “ecosystem services” – natural processes such as water recycling through rain and rivers, and pollination. Most of our energy still comes from fossil fuels, including from unconventional sources, like methane clathrates, tar sands, and fracked shale-gas deposits. Coal is still on the menu. We haven’t invested significantly in alternative energies. Globally, we also eat more meat and dairy products than we did in 2013. Consequently, emissions and temperatures are still rising fast. Droughts and floods are more frequent and claiming more and more lives. Ocean acidification is severe, and getting worse. The Arctic has not had ice during the summer for several decades. This article appeared in print under the headline “Pictures of Earth 2100” Correction: When this article was first published on 3 October 2013, the figures for global energy use in the four scenarios were mistakenly formatted without superscripts.