As COP26 came to a close on Saturday evening, its president, Alok Sharma, emphasized that as a result of nearly 200 nations noted its "pulse is weak.", the world had kept "1.5 degrees within reach" but
If the temperature of the planet rises above this number, scientists say there is a very high likelihood the consequences will be dire for many. Arctic ice sheets could begin to disappear and island nations end up underwater, while developing countries, already besieged by climate change, are likely to experience more extreme weather events more often. Bushfires will blaze more intensely, oceans will acidify and hurricanes will tear apart cities more often.
The world has been pursuing efforts to keep the global temperature rise below 1.5 degrees Celsius since the Paris Agreement was adopted in 2015, but earlier this year,, dealt a significant blow to those aspirations. Climate scientists say that, on our current trajectory, we're very likely to blow past 1.5 degrees Celsius of warming -- only a very, very small window of opportunity still exists to reach this target.
Projections back this up. During the first week of COP26, the International Energy Agency announced that if nations met their pledges "in full and on time," we might limit warming to 1.8 degrees Celsius (3.2 degrees Fahrenheit) by the end of the century. A week later, Climate Action Tracker suggested that meeting those same pledges put the planet on course for a rise of 2.4 degrees Celsius (4.3 degrees Fahrenheit).
These temperature increases have long been the focus of climate discussions but, alone, they fail to adequately reveal the impact humans are having on the Earth. The planet doesn't heat up evenly. The climate system governs, and is governed by, a diverse and complex series of interactions between seas and skies, forest and deserts, and oceans and ice caps.
As humans burn fossil fuels and release greenhouse gases, we are unbalancing the system -- causing unpredictable changes. We've already warmed the Earth by 1.1 degrees Celsius, and we're seeing some of those changes today.
How much will temperatures rise by the end of the century? In some ways that's the wrong question. Instead, we need to explore and center the broad, often outsized impacts those increases will have on the planet.
Global mean temperature
There are two critical temperature benchmarks you will always hear when it comes to limiting global warming: 1.5 degrees and 2 degrees Celsius.
The numbers refer to increases in the "global mean temperature" -- the average temperature across planet Earth. Scientists have known for generations that greenhouse gases retain heat and, in particular, carbon dioxide plays a key role in warming the Earth. Swedish scientist Svante Arrhenius noticed this fact more than a century ago.
Since Arrhenius, our understanding of this relationship has only been strengthened. For instance, scientists can drill into thick ice and retrieve rod-like cores, which function as planetary timestamps. Cores from the Antarctic demonstrate that global mean temperatures align well with carbon dioxide levels, stretching as far back as 800,000 years.
But global mean temperatures are a blunt instrument. Not necessarily a bad one -- there's value in projecting how much the average temperature across the planet is expected to rise based on how much carbon dioxide we pump into the atmosphere.
Organizations like the IEA and Climate Action Tracker examine how much carbon dioxide is expected to enter the atmosphere based on pledges made to cut back on emissions. They then make relatively simple calculations to understand how much temperatures will rise if pledges are met.
However, these calculations don't mean much on their own. For instance, if I turn the thermostat up by 1.5 degrees or 2 degrees Celsius in my apartment, I would probably struggle to notice the difference. Thinking about the planet in the same binary way misses a huge chunk of the story.
"These global numbers don't actually map onto risk," says Andy Pitman, director of the ARC Centre of Excellence for Climate Extremes in Australia. "There's no nuance in the conversations."
Global mean temperature benchmarks are useful insofar as they provide targets to be met. What really matters is understanding how different levels of warming might increase the likelihood of heatwaves, decrease ice coverage in the Arctic or reorganize ocean ecosystems across the world.
To work that out, you need extremely complex climate models.
The laws of physics
Climate models work, Pitman says, because they obey the laws of the universe.
"If you believe astronomy works, your microwave works, an ultrasound works," Pitman says, "if you believe those things work -- they work because of the laws of physics.
"It's those laws of physics that we build into our climate models to make projections."
Climate models have been in use since the 1960s and, despite what you may read on the internet, they've been fairly good at predicting some of the ways the climate will change as a result of increasing greenhouse gas emissions. "Even those primitive models in the '60s, the basic projections from those models have played out in time," says Matt England, an oceanographer at the University of New South Wales Climate Change Research Centre.
The complexity and resolution of the models have improved by magnitudes in the past 60 years, and there's dozens of models providing different projections for different climate phenomena.
Some climate variables map neatly to increasing global mean temperatures. Certain temperatures will see ice sheets melt and sea levels rise, for instance. But in practice, scientists can't plug in a warming of 1.5 degrees Celsius into a digital Earth and see the entire world the computer spits out. You'd need an unfathomable amount of computing power to simulate such a world. Pitman stresses these climate models are completely independent of the models used to project how much the world will warm on average by the IEA or Climate Action Tracker.
Climate scientists use dozens of different models to help predict changes to the climate that might play out at higher temperatures. They're not perfect and involve some level of uncertainty. "There's always going to be a bit of uncertainty because different models simulate the climate in slightly different ways," says Sarah Perkins-Kirkpatrick, a climate scientist at UNSW Canberra.
For instance, Perkins-Kirkpatrick studies heat waves, examining how warming at 1.5 or 2 degrees Celsius and more might affect the frequency and intensity of these events. At lower temperatures, the models show things get "a little bit worse," but as you begin to increase temperatures, the uncertainty gets a little bigger.
That's not to say things get better, but how much worse they get is still being deciphered.
The science is still very early in regards to predicting how 2 or 3 degrees Celsius warming might affect regions at smaller scales. It's hard to know exactly how weather patterns, ocean currents and land will change and the kind of feedback loops that will kick in. "There is no correlation at all between global mean temperature rise and that fingerprint of extreme events impacting a city or a region," Pitman says.
This is where the blunt instrument of global mean temperature rise fails to convey the urgency of the problem we've found ourselves in.
Too much warming
The science behind 1.5 degrees Celsius of warming is well understood. The impacts on different ecosystems, regions, weather patterns and adaptation at this increase have been intensely scrutinized by climate scientists, forming the basis of an IPCC special report published back in 2018.
The outlook in that report is grim -- and in the three years since its release, things haven't gotten any better. Even during a global pandemic, we've continued to burn fossil fuels and pump carbon dioxide into the atmosphere at an alarming rate.
"I think it's pretty much agreed across the scientific community that avoiding 1.5 degrees is virtually impossible now," England says.
The major emitters -- China, the US, India and the EU -- have all pledged stronger emissions reductions targets in the lead up to and during COP26. Writing in the journal Science on Nov. 5, climate policy scientists suggested the new pledges showed a "strengthening of ambition through 2030" with a "stronger near-term foundation to deliver on the long-term goals of the Paris Agreement."
But this analysis relies on the latest pledges being met in full and on time. That's not a given, by any stretch. "The uncertainty for the future of the planet is locked up in where humanity will take our emissions," England says.
The idea we might still limit warming to 1.8 degrees Celsius was viewed as "optimistic" in the first week of COP26. A week later, 2.4 degrees of warming was described as "catastrophic."
The unfortunate truth is that limiting the mean temperature rise to 1.5 degrees is already too much warming. That doesn't mean we shouldn't do everything in our power to limit warming to 1.5 degrees, but we have to be realistic. This is no reason to give up. In fact, it provides even more impetus to rapidly decarbonize -- the alternative is far worse. "Very soon," Perkins-Kirkpatrick says, "2 degrees will no longer be possible."
Those numbers signify more than summers becoming slightly hotter in San Francisco, Sydney or Svalbard. The planet's response will differ across its oceans, ice and land. Those numbers signify potentially irreversible changes in the Earth's climate. That's what the climate models tell us.
So how much warming can we expect to see by the end of the century? How much will temperatures rise?
Whatever the number, it's already too much.