How hot could it get?
How hot could it get?
“The last time humans experienced [carbon dioxide] levels this high was…never. Humans didn’t exist.” – Dr Peter Gleick
- The last time Earth’s atmosphere contained the amount of carbon dioxide (CO2) present today (>400ppm)—during the Pliocene Epoch of 2.6 to 5.3 million years ago—sea levels were 10 to 20 metres higher and global temperatures were an average 2 to 3°C warmer. Due to polar amplification, Antarctica was a plant-covered oasis up to 14°C warmer.
- Even if countries act on their Paris climate agreement pledges to reduce emissions so that temperatures do not go beyond 1.5°C, the most recent UN Report states that we are on track for warming of more than 3°C by the end of the century (Video 2).
- If we stay on our current track and double CO2 by 2060, ‘the most likely temperature range will be 2.6°C to 3.9°C
- During the Cretaceous 145 to 65 million years ago, one of the warmest periods on the planet, atmospheric CO2 was around 1,000ppm, the average ocean temperature 35°C, there was no ice, anywhere, and temperatures at the poles were 14°C in winter (today, winter temperatures are around -49°C) (See: how we know about past climates)
Does it matter if it gets warmer?
“Ecosystems, species, wild populations, local varieties and breeds of domesticated plants and animals are shrinking, deteriorating or vanishing. The essential, interconnected web of life on Earth is getting smaller and increasingly frayed. This loss is a direct result of human activity and constitutes a direct threat to human well-being in all regions of the world.” – Prof. Settele, Global Assessment Report on Biodiversity and Ecosystem Services
At the end of the Last Glacial Maximum 14,500 years ago, the amount of CO2 in the atmosphere was around 260ppm. It slowly increased to 277ppm until 1750 as agricultural emissions slowly grew. Then the Industrial Revolution began, and we started burning large quantities of fossil fuels, which produces greenhouse gasses.
By April 2021, atmospheric CO2 had passed 421ppm and global temperatures were 1.15°C warmer than pre-Industrial levels. Adding so many more greenhouse gasses to the atmosphere in just a few hundred years has changed the climate too rapidly for many plants and animals to adapt.
Most of the plants and animals alive today—including humans, which have only been around for about 200,000 years—evolved to live in an atmosphere where the quantity of greenhouse gasses was just right to keep in enough heat, but not too much. Many plants and animals can’t cope with even small changes in temperatures, so they’re going extinct or trying to move to places that are cooler; either to higher latitudes (towards the poles) or higher altitudes (up mountains). This is disrupting entire ecosystems, bringing pests and diseases into new areas that aren’t adapted to them, and threatening multiple life-supporting ecosystem services that humans are utterly dependent upon—including food and water.
“If our experiment was to see if we could recreate the Pliocene, we’re doing a really good job! We’re spot-on with that. And for our planet to create a Cretaceous, if we keep going for another 50 years, we’re fine!” – Prof. Martin Seigert, Grantham Institute (Video 1).
Video 1: ‘The Pliocene: The Last Time Earth had over 400 ppm of Atmospheric CO2’; 3.5 hour recording of lectures: The Grantham Institute and Royal Meteorological Society National Meeting, April 2019.
As water expands with rising temperatures, sea levels rise. In addition, as ice caps melt, sea level rises even more. Land that is currently home to 300 million people globally will flood at least once a year by 2050.
“In New Zealand, 72,000 people are currently exposed to present-day extreme coastal flooding, along with about 50,000 buildings worth $12.5 billion. The risk exposure increases markedly with sea-level rise, particularly during the first metre of rise.
“There is near certainty that the sea will rise 20-30 cm by 2040. By the end of the century, depending on whether global greenhouse gas emissions are reduced, it could rise by between 0.5 to 1.1m, which could add an additional 116,000 people exposed to extreme coastal storm flooding.” – NIWA (see also Video 3)
Video 3: NIWA presentation on the release of its 2019 reports outling the risks of flooding from rain (pluvial), rivers (fluvial) and rising sea levels.
What’s the difference between 1.5°C, 2°C or a bit more? It’s just half a degree, right?
These are average global temperatures, not maximum temperatures (Fig. 2). Most of the planet is ocean, and while it’s also warming, it has a massive volume so the changes are slower.
One way to think of a half a degree difference is: what happens to ice when temperatures permanently change from -0.25°C to +0.25°C? Ice caps and glaciers melt and sea levels rise. Half a degree is also enough to change the way plants and animals behave, with major implication for biodiversity and agriculture. It also means the atmosphere can carry another 3.5% of moisture. And that means more powerful storms and floods.
Carbon Brief has extracted data from around 70 peer-reviewed climate studies to show how global warming is projected to affect the world and its regions, based on 1.5°C, 2°C, and even higher warming (based on our current emissions pathway). Figures 3 and 4 are examples. Click on any image to see the full range on Carbon Brief’s website..
References and further reading
- Royal Meteorological Society: The Pliocene: The Last Time Earth had >400 ppm of Atmospheric CO2
- NCIDSC (National Snow and Ice Data Centre): Climate change in the Arctic
- 2020: UN Emissions Gap Report
- 2020: Sherwood et al; An assessment of Earth’s climate sensitivity using multiple lines of evidence AUG: Reviews of Geophysics
- Carbon Brief guest post by the authors, explaining the climate sensitivity research paper; Why low-end ‘climate sensitivity’ can now be ruled out
- Science magazine explaining the same paper; After 40 years, researchers finally see Earth’s climate destiny more clearly
- 2020: Prof. Shulmeister University of Canterbury guest post on sciblogs: What the world was like the last time carbon dioxide levels were at 400ppm
- 2020 NOAA: Global Climate Report
- 2020 IPBES (Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services) Global Assessment Report on Biodiversity and Ecosystem Services (all chapters)
- Deep South Challenge New Zealand
- 2019 Smithsonian Magazine: CO2 Levels Are as High as They Were Three Million Years Ago
- 2019 The Grantham Institute: The Pliocene: The Last Time Earth had over 400 ppm of Atmospheric CO2 (Youtube recording of papers presented at this meeting)
- 2019: Kulp & Strauss; New elevation data triple estimates of global vulnerability to sea-level rise and coastal flooding, Nature Communications 10, Article number: 4844
- 2019 BBC article: Climate change: Warning from ‘Antarctica’s last forests’
- 2014 IPCC AR5 Climate Change 2014: Impacts, Adaptation, and Vulnerability