Limiting global warming to 1.5 degrees versus 2 has big benefits, the IPCC says

Half a degree can make a world of difference.

If Earth warms by just 1.5 degrees Celsius over preindustrial times by 2100, rather than 2 degrees, we would see fewer life-threatening heat, drought and precipitation extremes, less sea level rise and fewer species lost.

Those findings are detailed in a report, a summary of which was released October 8, by the Intergovernmental Panel on Climate Change, or IPCC, following its weeklong meeting in Incheon, South Korea. “This will be one of the most important meetings in the IPCC’s history,” Hoesung Lee, a climate economist at Korea University in South Korea and current IPCC chair, said in his opening address October 1. 

To compile the report, the scientists sifted through more than 6,000 papers probing the impact of a global temperature hike of 1.5 degrees, sometimes working through the night, says Natalie Mahowald, a climate scientist at Cornell University and one of the report’s coauthors. But the long hours were worth it: The report’s message is compelling and urgent, she says. “Such a small change in temperature will have big impacts on people.”

Three years ago, in 2015, 195 nations signed onto the Paris agreement to curb greenhouse gas emissions sufficiently to limit global warming to “well below” 2 degrees by 2100 (SN: 1/9/16, p. 6). Getting all the delegates on the same 2-degree-warming page was a hard-won victory. But many scientists have warned that the 2-degree target isn’t stringent enough to prevent major environmental changes affecting everything from sea level rise to water scarcity to habitat loss. During the Paris talks, more than 100 nations — including many of those most vulnerable to climate change, such as the island nation of the Maldives and drought-stricken Angola — called for a lower warming target of 1.5 degrees.

At the time, Lee noted in his Oct. 1 address, scientists knew relatively little about how to compare the risks of a 1.5-degree-warmer world with a 2-degree-warmer world. So, as part of the decision to adopt the Paris agreement, the nations invited the IPCC to prepare a report assessing those impacts.

As it turns out, the differences are stark between the two warming targets, as outlined in the new report, titled “Global Warming of 1.5° C.”  In addition to fewer heat, rain and drought extremes, the impact on future sea levels would be significant. A half a degree less warming means about 0.1 meters less sea level rise on average by the next century. As a result, at least 10 million fewer people would be exposed to such risks as flooding, infrastructure damage and saltwater intrusion into freshwater resources, the report found.

Somewhere between 1.5 degrees and 2 degrees, the planet’s great ice sheets may become increasingly unstable, further increasing the potential for sea level rise. And, in the 1.5-degree warming scenario, the Arctic Ocean is projected to be ice-free during the summer only once per century. That would happen once a decade under the 2-degree scenario.  

As for the planet’s other denizens, a lower temperature increase would mean less risk of habitat loss for many insect, plant and animal species compared with a full 2 degrees of warming, the report notes (SN: 6/9/18, p. 6). And other climate-related risks to these species, including forest fires and the spread of invasive species, would be less under that lower warming threshold.

Much of the data analyzed in the report has been published in scientific journals over the last two years, and wasn’t available when the Paris agreement was signed. The London-based website Carbon Brief published an interactive infographic on October 4 that summarizes the results of 70 such 1.5-degree studies that show the impacts of warming targets on everything from future sea level rise to heatwaves to hurricanes.

Despite building a case for a lower temperature target, the trick will be how to get there. In 2017, the Paris accords faced a major setback when President Donald Trump announced that the United States, a major contributor to the greenhouse gases that drive warming, would pull out of the agreement. Achieving an even more stringent target seems particularly daunting.

The IPCC report examines various possible paths that scientists have examined to limit the environmental impacts of warming. Among the variables considered in these paths are when emissions are projected to reach net zero, when the amount of carbon released to the atmosphere is balanced by the amount that is being removed. Another variable is how many more emissions will be allowable in the meantime — a concept known as the carbon budget.

But almost all of the projected pathways to 1.5 degrees have one thing in common, says Zeke Hausfather, a climate scientist with Carbon Brief: They overshoot that temperature threshold somewhere around 2050. “They all exceed it — and then back down,” he says.

To overshoot the mark by only a small amount, or not at all, requires reducing emissions by about 45 percent relative to 2010 levels by the year 2030, and reaching net zero around 2050, the IPCC report notes. In comparison, to get to “below 2 degrees Celsius,” emissions must decline by about 20 percent by the year 2030 and reach net zero by about 2075.

Barring such early, deep cuts, it will take “negative emissions” to bring the temperatures back down after overshooting the mark mid-century. Negative emissions are, essentially, a hoped-for reduction in emissions due to future technologies that will be able to remove enough carbon dioxide from the atmosphere to reverse the greenhouse effect.

Those technologies, such as carbon capture and storage, are not yet commercially viable. And reversing the effects of the greenhouse warming is not so straightforward: “By and large, it’s generally true that there’s a linear relationship between warming and carbon dioxide in the atmosphere, as long as both are increasing,” Hausfather says. “But once you start sucking carbon out of the atmosphere, that linear relationship breaks. You need more negative emissions to reduce temperatures than positive emissions to increase them.”

It’s uncertain how — or if — policy makers will be able to use the findings to reshape the climate accords. During the 2015 Paris talks, a proposed 1.5-degree target was met with strong resistance from nations that would need to be on board, particularly China.

And President Trump’s administration gave a hint of its reaction when, in July, it released an environmental impact statement on the White House’s plan to freeze federal fuel efficiency standards for cars and light trucks built after 2020. With current levels of greenhouse gas emissions, global temperatures are currently on track to rise by about 4 degrees compared to preindustrial times by 2100 — and freezing the standards will further increase those emissions, the report acknowledged. But the study recommended the freeze anyway, stating that moving away from fossil fuels to make deep cuts in carbon emissions would require innovations that are “not currently technologically feasible or economically feasible.”

The challenges may seem insurmountable. But this IPCC report — and the two years of intense research that led up to it — has also forced scientists to reassess some assumptions about what’s possible, says Kaisa Kosonen, a Helsinki-based climate policy adviser at Greenpeace International who traveled to South Korea for the meeting. Including certain factors, such as better energy efficiency in the future, can lead to “results you didn’t know even existed,” she says. “I’ve been inspired by how much optimism there still is among scientists.”

Indeed, one of the key messages from the report is that holding warming to 1.5 degrees “is not impossible,” Mahowald says. “But it will require really ambitious efforts, and the sooner the better. We have to start cutting emissions now. We have to be very ambitious on sustainable energy and sustainable agriculture,” she says. And, she notes, achieving the goal will require people to undergo behavioral changes as well, from energy conservation to changes in diet.

But people would also face huge adjustments in a world that’s 2 degrees warmer, or even higher, Mahowald warns. So despite the challenges, “it still might be easier to reach 1.5 than to adapt to those higher temperatures.”

Editor's note: This story was updated October 8, 2018, to include reaction to the report.