‘A deeply troubling discovery’: Earth may have already passed the crucial 1.5°C warming limit

• The worrying findings, based on temperature records contained in sea sponge skeletons, suggest global climate change has progressed much further than previously thought.
• Human-caused greenhouse gas emissions drive global warming.
• To date, estimates of upper ocean warming have been mainly based on sea-surface temperature records, however these date back only about 180 years.
• Earth may in fact have already reached at least 1.7°C warming since pre-industrial times – a deeply troubling discovery.

Getting a gauge on ocean heat

• Oceans cover more than 70% of Earth’s surface and absorb an enormous amount of heat and carbon dioxide.
• Global surface temperatures are traditionally calculated by averaging the temperature of water at the sea surface, and the air just above the land surface.
• This makes it more difficult to accurately reconstruct stable baseline ocean temperatures.
• But what if there was a way to precisely gauge ocean temperatures over centuries in the past?

Studying a special sponge

• But they grow at a much slower rate and can live for many hundreds of years.
• This means sclerosponges can provide a detailed diary of sea temperatures, down to a resolution of just 0.1°C.
• We studied the sponge species Ceratoporella nicholsoni.

• We looked at temperatures going back 300 years, to see whether the current time period which defines pre-industrial temperatures was accurate.
• The sponge records showed nearly constant temperatures from 1700 to 1790 and from 1840 to 1860 (with a gap in the middle due to volcanic cooling).

What does this mean for global warming?

• It shows human-caused ocean warming began at least several decades earlier than previously assumed by the IPCC.
• Long-term climate change is commonly measured against the average warming over the 30 years from 1961 to 1990, as well as warming in more recent decades.
• Add to that the average 0.8°C global warming from 1990 to recent years, and the Earth may have warmed on average by at least 1.7°C since pre-industrial times.
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We must act now

Our revised estimates suggest climate change is at a more advanced stage than we thought. This is cause for great concern. It appears that humanity has missed its chance to limit global warming to 1.5°C and has a very challenging task ahead to keep warming below 2°C. This underscores the urgent need to halve global emissions by 2030.
Malcolm McCulloch receives funding from the Australian Research Council.