We have some idea of what the future may look like under climate change, and now, thanks to new research, we have a better idea of when. The research, published today in Nature, shows that the world’s climate will have fundamentally changed by 2050 if we do nothing to slow greenhouse gas emissions.

Experts warn of serious disruptions to ecological and social systems, particularly in the tropics and low-income countries.

The study compared two scenarios: a business-as-usual scenario with no emissions reductions, and a scenario with moderate emissions reduction. High emissions reductions weren’t included because they are currently considered unfeasible.

If we don’t act to reduce emissions, most of the world will experience unprecedented climate change by 2047. If we act to moderately reduce emissions the date is pushed back 20 years to 2069.

Unprecedented climate change means climate variables - such as temperature, evaporation and precipitation - that have moved permanently outside the range of historical variation.

For the study future temperature scenarios were compared to the historical period 1860 - 2005. The earth has likely already experienced single years already outside the range of historical variations. The study calculates the date at which every year will be outside historical variation.

Modelling of different climate variables showed different results, with temperature most consistent across the globe. Ocean acidity is already outside the bounds of normal variation, having passed the threshold in 2008.

The research found the tropics will be the first regions of the world - as early as 2020 - to experience unprecedented climate change, probably because the tropics currently experience little natural variation in temperature.

The study particularly considers effects on tropical people and biodiversity. Five billion people currently live in areas that will see dramatic change by 2050 without climate action, most in low-income countries.

Most of the world’s animals and plants are also concentrated in tropical hotspots such as rainforests and coral reefs. According to modelling these areas are likely to experience unprecedented climate change much earlier than the global average.

Australian cities are likely see change earlier than the global average, with Sydney to see unprecedented change by 2038. Brisbane and Perth follow in 2042, and Melbourne and Canberra in 2045.

Ryan Longman, PhD candidate at University of Hawai'i and an author on the study, said the findings are a call to action.

“The worst thing we can do is nothing. We are hoping that our work connects people to the issue in a way that helps them accept the changes that need to be made.”

He warned that low-income countries, most of them in the tropics, will suffer most despite relatively low contributions to greenhouse gas emissions.

“By implementing mitigation efforts we may buy these countries the time they need to cope with these changes.”

New ways of seeing

Dr Sarah Perkins at University of New South Wales said the study is a “new methodology” giving us a fresh perspective on an old problem.

The findings also provide a new insight into tropical regions, which have gone under the radar in other climate analyses. Dr Sophie Lewis at University of Melbourne explained:

Tropical areas aren’t usually represented in metrics that represent absolute climate change, for instance the 2C guardrail that’s often talked about. This paper identified the tropics as important because although 2C might correspond to a small shift in climate, the rapid change could be difficult for biological and humans systems to cope with.

Professor Stephen Williams at James Cook University warned that the results presented in the study reflect changes in average climatic conditions, not extremes.

“When your variability goes so far outside normal variability what you’re really doing is increasing the frequency and intensity of extreme climate.

"It’s not so much the increase in the mean that matters, or even how far outside the current mean, it’s the fact that we’re going to get record-breaking heatwaves, record-breaking droughts, record-breaking floods. That is probably the single biggest impact on anything.”

Sarah Perkins also cautioned focusing on a single year instead of the bigger climate trend.

“The average time of emergence from historical variation is 2047, with an 18 year margin either side. If we focus on one year the study could be misinterpreted.”

Tropical biodiversity

The tropics - home to more animals, plants and other lifeforms than any other region on Earth - are uniquely vulnerable to the changes forecasted by the study.

Stephen Williams explained:

Even though the tropics are the hottest place on earth they’ve actually got the least variability in temperature. They’re the most stable in terms of rainfall and temperature across the year and from year to year. That means animals and plants have quite a narrow range of tolerance.

Prof Williams said reptiles, amphibians and invertebrates are particularly vulnerable because they are already close to their limits of temperature tolerance.

Parts of the tropics are already stressed thanks to human activities such as logging and land clearing. The combination of human pressure and rapid climate change would likely lead to dramatic changes in tropical ecosystems.

To read more on changes to tropical ecosystems, see analysis from Professor Stephen Williams here