The sun hangs in an orange sky over London.

The study suggests that the world has warmed above the 1.5⁰C threshold thought to limit the worst impacts of climate change. Image © Sven Hansche/Shutterstock. 

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Earth has already passed 1.5⁰C of warming, controversial study claims

Nine years ago countries around the world committed to try and limit climate change to 1.5⁰C to limit the worst effects of a warming world.

But new research is claiming that this threshold was breached years before countries signed the agreement. Other scientists, however, have questioned the findings.

Has the world already passed a critical level of global warming? A new paper claims so.

The study, published in the journal Nature Climate Change, suggests that ocean temperatures have already warmed by at least 1.5⁰C since the 1860s. Beyond this threshold, climate change will become increasingly extreme and alter Earth’s climate in ways that can’t easily be undone.

Professor Malcolm McCulloch, the study’s lead author, says, ‘Our results show that the era of industrial warming started earlier than we thought, so global mean surface temperatures are almost half a degree higher than currently accepted estimates.’

‘Half a degree is a dramatic increase in temperature, which has a number of significant impacts. It means that, without major reductions in emissions, we will reach 2⁰C of warming by the late 2020s.’

‘The time to reduce damaging emissions in has been brought forward by at least a decade, requiring a major change in how we think about global warming.’

Other climate researchers, however, aren’t convinced by the paper’s conclusions. While the study’s approach by relating levels of different elements in sponge skeletons to temperature changes allowed the authors to look at a longer time period, the use of samples just from the Caribbean means this may be a regional, rather than global, trend.

Professor Daniela Schmidt, a researcher investigating marine climate change who was not involved in the study, says, ‘Reconstructing baselines for past climates is fundamentally important so that we understand the uniqueness of our current climate crisis. Skeletons and shells of fossils are an important archive for these changes, and form the basis of this analysis.’

‘Such studies are challenging, due to their small sample sizes, regional settings, and the strong biological modification of the climate signal by the organisms themselves, which results in uncertainties.

‘The claim that we might have overshot 1.5⁰C is being made rather confidently, when in fact there are several uncertainties and limitations in this study which must be acknowledged.’

Sunset over Bristol harbour, with the masts of the SS Great Britain rising into the sky.

Ocean temperature measurements were first widely taken from the 1850s by boats like the S.S Great Britain (pictured), but these may not be early enough to provide an accurate baseline for Earth's climate. Image © Sion Hannuna/Shutterstock.

Tracking global temperatures

Being able to accurately track the advance of global warming is critical to work out how much time we have to turn things around. While the world may be covered in sensors that allow us to work out temperatures today relatively easily, the problem is working out how warm Earth used to be.

At the time of the industrial revolution, taking regular temperature records was uncommon. This only began to change in the 1850s, when ships travelling the world started to take measurements to help them better navigate the seven seas.

While these measurements form the basis of our current climate estimates, they’re not an exact science. The equipment the sailors used was rudimentary, while different ships often used different ways to take measurements.

There was also a lot of volcanic activity in the 1800s, which complicates things further. The eruption of volcanoes like Tambora and Krakatau threw volcanic ash into the atmosphere and temporarily cooled the world by about half a degree.

To get around these issues, the current study turned to sponges in the Caribbean.

A large circular caldera with rocky sides reaching down towards a lake in the centre.

The eruption of Mount Tambora in 1815 is thought to have been the largest in human history, leading to 'the year without a summer' due to its climate impacts. Image © MudaCom/Shutterstock. 

These animals build their skeletons by extracting minerals such as calcium and strontium from seawater, with the ratio of strontium to calcium in a sponge changing based on temperature. Lower ratios suggest that the water is warmer, whilst higher ratios imply the temperatures are cooler.

As sponges grow like trees, adding new layers on top of the old, these ‘rings’ can be sampled to see what the temperature was like at the time that tissue was growing. Because sponges can live for hundreds of years, this can allow scientists to peer back in time.

While the rings don’t give a precise temperature, the researchers believe that the changes in the strontium to calcium ratio are in proportion to the changes in the global average temperatures. This would mean that the sponges in the Caribbean contain a record spanning over 300 years.

‘This record goes back almost 150 years before global temperatures started to increase,’ Malcolm says. ‘There are indications in the record that this is a global proxy, with the sponges all showing the same period of global cooling caused by volcanic activity, which is in line with what we expect.’

Their results, based on samples of multiple sponges from different depths, suggest the world was already warming in the late 1800s. This means that the current baselines from other records start in a world that was already warmer than it should have been, and so we are currently underestimating the level of global warming that has occurred.

These new calculations suggest that the world already passed 1.5⁰C of warming by around 2010, with temperatures now around 1.7⁰C above pre-industrial levels. The authors have called for countries to urgently accelerate their net zero plans to limit global warming to less than 2⁰C.

Clouds hang above the tip of the island of St Croix, with blue seas surrounding it.

The sponges were sampled from off the coast of the islands of Puerto Rico and St Croix. Image © Dondre Richards/Shutterstock. 

Has the world already warmed by 1.5⁰C?

While other scientists have welcomed the new data presented by the paper, they’re concerned by how confidently its findings are being presented. Extraordinary claims require extraordinary evidence, and many aren’t convinced that the world has breached the 1.5⁰C barrier.

One area of concern is that the study only looks at a single species of sponge found exclusively in the Caribbean. As different parts of the world are warming at different rates, it could mean that the study shows how this region is changing but not the planet as a whole.

Dr Gavin Schmidt, Director of the NASA Goddard Institute for Space Studies, was not involved in the research. He says, ‘Estimates of the global mean temperatures before 1850 require multiple proxies from as wide a regional variation as possible. Therefore, claims that a single set of records can confidently define the global mean warming since the pre-industrial are probably overreaching.’

‘There is a real uncertainty in what the mid-nineteenth century temperatures were compared to the modern period, around 0.2⁰C even in the instrumental record, and so that complicates our ability to make definitive statements about the crossing of the 1.5⁰C level.’

Other scientists have also questioned how reliable the sponges currently are as a source of climate data.

Professor Kate Hendry, a researcher from the British Antarctic Survey who was not part of the research, says, ‘There are considerable complexities surrounding the use of sponge skeleton chemistry as archives of past ocean change.’

‘We need to know far more about how these animals make their skeletons, and exactly how their chemistry relates to ambient temperatures – something we don’t have a good handle on now. We need a better understanding of these proxies before we can use them to make important statements about the state of the climate with confidence.’

Kate suggests that growing samples of these sponges under controlled temperatures in a lab would allow researchers to find out more about their growth and be more confident in the findings.

One thing the authors and other scientists can agree on, however, is that greater climate action is needed now.

‘The absolute degree of warming will always depend on which baseline you use,’ Daniela says. ‘The absolute number should not be the focus of the discussion, though.’ 

‘While the Paris Agreement strongly focused on 1.5⁰C, we know that impacts increase with every increment of warming. Even if we do miss a target, we should not lose all hope, but instead increase our efforts to limit emissions, mitigate current warming, and reduce the risks via adaptation.’