Which Climate Scenarios Limit The Earth’s Warming To 1.5 Degrees Celsius?

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At the UN Climate Change Conference late last month, world leaders reaffirmed the 1.5 C goal of carbon reductions that’s been accepted as necessary to mitigate the climate crisis. But without dramatic action — action the leaders at COP27 did not commit to taking — lessening global warming to livable levels most likely will not be possible. Instead, governments around the world need to embrace viable climate scenarios that exist and point to a less hot future. These paths rely on negative emissions technologies to reach net zero around mid–century and then go farther.

So many conflicting factors are at play, however, to diminish carbon reductions globally. How can economic efficiency, ethics and equity, interlinked technological and social transition processes, and socio-political frameworks work together to solve our existential crisis?

Is there actually hope to reign in the Earth’s warming?

The Sixth Assessment Report of the of the UN Intergovernmental Panel (IPCC) on Climate Change in 2020 indicated that the total net anthropogenic GHG emissions have continued to rise during the period 2010–2019, as have cumulative net CO2 emissions since 1850. Average annual GHG emissions during 2010–2019 were higher than in any previous decade.

Emissions reductions in CO2 from fossil fuels and industrial processes (CO2-FFI), due to improvements in energy intensity of GDP and carbon intensity of energy, have been less than emissions increases from rising global activity levels in industry, energy supply, transport, agriculture, and buildings.

“This is a climate emergency,” says United Nations Secretary General António Guterres, adding that wealthy economies and corporations “are not just turning a blind eye; they are adding fuel to the flames. They are choking our planet, based on their vested interests and historic investments in fossil fuels, when cheaper, renewable solutions provide green jobs, energy security and greater price stability.”

We all know it: the world’s most important climate goal is limiting the Earth’s warming to 1.5 degrees Celsius (2.7 Fahrenheit). If all human emissions of heat-trapping gases were to stop today, Earth’s temperature would continue to rise for a few decades as ocean currents bring excess heat stored in the deep ocean back to the surface. NOAA explains that, once this excess heat radiated out to space, Earth’s temperature would stabilize, with the additional warming from this “hidden” heat unlikely to exceed 0.9° Fahrenheit (0.5° Celsius). With no further human influence, natural processes would begin to slowly remove the excess carbon dioxide from the atmosphere, and global temperatures would gradually begin to decline.

It’s clear that we need to transition all sectors of our economy away from fossil fuels that emit carbon, increase our use of clean energy sources like wind and solar, harness the power of nature to capture carbon, and deploy technologies that capture and store carbon. But what’s the right mélange of solutions to prevent a desperate future on an overheated planet?

Sorting through Climate Scenarios

If you’re a team looking for solid research analysis, there are few better places to turn than the Potsdam Institute for Climate Impact Research in Germany. Its mission is to advance the scientific frontier on interdisciplinary climate impact research for global sustainability while also contributing knowledge and solutions for a safe and just climate future. They integrate the latest understanding of the Earth system with the assessment of climate risks and exploration of policies and pathways towards a manageable climate future.

The Washington Post examined over 1,200 different scenarios for climate change over the coming century, based on the models produced by the world’s leading climate scientists. Working with experts from the Potsdam Institute, the Post explored the central features of climate scenarios — how fast the world embraces clean energy, how quickly we can remove greenhouse gases from the atmosphere — and looked at how these, in turn, affect the planet’s temperature over the course of the century.

They also considered findings in the key 2022 IPCC report. Out of more than 1,200 scenarios — some with temperatures rising as high as 5° C above preindustrial levels — 230 paths leave our planet below 1.5° C before the end of the century.

The results show a world that keeps inching closer to catastrophic climate change. But many climate scenarios do point a way toward a less hot global future. The scenarios help show us what needs to be done — and what we can still do.

Two scenarios depict a planet, in the year 2100, below 1.5 C of warming.

Scenario One, a High Overshoot: This scenario shows the world plodding away for decades above 1.5° C. This is an incredibly unsettling prospect because of dangerous tipping points and even calamities such as the irreversible loss of the West Antarctic ice sheet. In high overshoot, the Earth’s temperature leaps far above 1.5° C before coming back down again.

Scenario Two, a Low Overshoot (or none at all): The world will have to be removing more carbon dioxide from the atmosphere than it is putting in — “net negative.” And that will require the wide-scale deployment of nascent carbon capture technologies to remove what is already present, storing it underground, and also massive reforestation and other efforts to store carbon in the land itself.

The researchers and their international colleagues filtered scenarios based on the kinds of future developments they anticipate and the speed of progress they show happening within ranges of “speculative,” “challenging,” or “reasonable,” based on progress by 2050:

1. Carbon dioxide removal and storage underground
2. Carbon dioxide removal using land
3. Carbon intensity reductions
4. Changing energy demand
5. Fewer methane emissions

What makes these scenarios work?

• Dramatic carbon removal from the atmosphere: Such climate scenarios require subtraction of over 7 billion tons per year from the atmosphere by 2050 and a huge scale up of interventions like carbon capture and storage, which only has an estimated capacity of about 43 million tons per year today. This might be underground or in forests and agricultural landscapes.
• A combination of renewables and vastly expanded energy efficiency: Many climate scenarios require the carbon intensity of energy use — how much CO2 is emitted per unit of energy consumed — to decrease by over 80% by 2050. This would require total or a near total phaseout of fossil fuels, widespread electrification of the world’s energy systems, and major fuel shifts in transportation to electric vehicles or the use of other fuels such as hydrogen or biofuels, among other innovations.

Final Thoughts

There is, of course, another scenario. Global North governments could continue to hem and haw about their responsibilities, so  that temperatures significantly overshoot 1.5° C during the century. Temperatures expected to be between 1.6° C and 1.8° C above preindustrial levels through the 2040s, 2050s, and 2060s in most of the scenarios would raise the odds of unexpected climate catastrophes.

Of course, not everybody agrees with these models or the cutoffs imposed by the Potsdam Institute researchers. Some experts are more optimistic about technology and humanity’s ability to innovate. Others point out that it is easy to imagine countries failing to achieve what is necessary to stay below 2° C at all.

In the end, these are simply well-informed models of how the world will work. What’s more, we still have a limited understanding of how the climate system will respond to emissions.

Without making explicit the consequences of our current pathways, however, global agreements will continue to be thin in application. With follow-throughs to commitments, the Post researchers envision that keeping warming this low will help save the world’s coral reefs, preserve the Arctic’s protective sea ice layer and could avoid further destabilizing Antarctica and Greenland, staving off dramatic sea level rise.

It’s a marvelous goal and one which the Global North governments need to embrace.