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Business News/ Science / 1.5 Degrees: A Tiny Number With a Global Effect

1.5 Degrees: A Tiny Number With a Global Effect


The goal to limit warming appears elusive as the COP28 summit opens.

Efforts were under way to extinguish a wildfire near Athens in JulyPremium
Efforts were under way to extinguish a wildfire near Athens in July

Nearly 200 nations and business leaders pledged eight years ago to keep Earth from warming above 1.5 degrees Celsius. That goal has proven difficult to meet.

The 1.5 C threshold isn’t a random number. It has been embedded in policies that affect how we heat and cool our homes, what kind of cars we drive, and how nearly all businesses operate. But even as the U.S. and many other nations ramp up cleaner renewable solar and wind power, overall emissions of carbon dioxide keep going up. As a result, the Earth’s temperature continues to rise, and 2023 is virtually certain to be the warmest on record.

On Thursday, world leaders and companies will convene in Dubai at the COP28 climate summit where negotiators are pushing big nations to make good on promises to slash industrial emissions to keep the Earth from warming above 1.5 degrees Celsius—about 2.7 degrees Fahrenheit.

Here’s what to know about the 1.5 C goal:

Why is 1.5 C important?

Climate scientists—supported by research—chose the number more than a decade ago as a speed limit for rising global temperatures beyond which some effects of climate change start to become irreversible. The number refers to the difference in the average global surface temperature between today and the preindustrial climate of the late 1800s. Today, that difference is about 1.1 degrees C or 2 degrees Fahrenheit.

Why did the United Nations choose 1.5 C for its global climate treaty?

The 1.5 C goal was initially set as a political target that was backed up by research from the Intergovernmental Panel on Climate Change, according to Anne Olhoff, co-editor of the United Nations Emissions Gap Report, which was released Nov. 19.

Scientific studies beginning in the 1970s predicted environmental consequences if the atmosphere warmed between 1 and 2 degrees Celsius. In the years since, scientists have used climate models to predict what would happen at various levels of warming.

The 1.5 C target was introduced at the 2009 Copenhagen climate summit when a group of small island nations demanded wealthier countries limit warming, fearing catastrophic sea-level rise that would devastate their low-lying countries.

In 2010, the U.N. officially set 1.5 C as a long-term goal for all nations. In 2015, 195 countries signed a pact known as the Paris agreement, to hold temperatures below 2 degrees C and pursue efforts to stay within 1.5 C.

Since Paris, nations have established road maps on how they plan to achieve that goal. These plans include using carbon-free sources of energy, encouraging drivers to buy electric vehicles, and for some industries, capturing and storing carbon-dioxide emissions underground. In 2021, President Biden promised to cut U.S. emissions by 50% over 2005 levels by 2030 and achieve “net-zero emissions" by 2050.

Is 1.5 C still a realistic goal?

Several scientists say 1.5 C is improbable. Recent studies forecast that the 1.5 C threshold will be surpassed on a permanent basis within the next decade. The new U.N. emissions gap report estimates that countries will have to slash carbon emissions by 42% by 2030 to have a chance at keeping within the 1.5 C limit. That seems unrealistic since emissions continue to rise globally by about 1% each year, according to an analysis by the Rhodium Group, an independent consulting firm.

More government regulations won’t get the world to meet these climate goals; instead, industry has to see business opportunities for investing in cleaner energy and transportation, said John Christensen, co-author of the U.N. emissions report and director of the U.N. Environment Program’s Copenhagen Climate Center.

“This has to be done in a way so it’s attractive to the private sector," said Christensen. “There’s not going to be enough public money to do this."

How is the global average temperature measured?

To take the planet’s temperature, scientists collect readings from more than 10,000 land-based weather stations, as well as sensors on oceangoing drifters, moored buoys and shipboard instruments. The information is fed into climate databases managed by several national agencies and then compiled into long-term temperature records.

The National Oceanic and Atmospheric Administration, along with the National Aeronautics and Space Administration, the U.K.’s Meteorological Office and the European Union’s Copernicus Climate Change Program derive their own temperature data sets.

“Multiple groups do it in different ways, but we still come up with very comparable rates of long-term climate change," said Russell Vose, monitoring and assessments branch chief at NOAA’s National Centers for Environmental Information. “That is what gives us confidence."

Vose said the global measurements take into account the localized warming that occurs around cities, known as the urban heat-island effect. They also factor in gaps in past data when there weren’t as many weather stations as there are today.

How do the oceans affect global temperatures?

Even though the earth’s surface is 70% ocean, land areas have warmed faster, according to NOAA. The ocean helps regulate Earth’s climate and has absorbed more than 90% of the excess heat and almost 30% of the excess carbon dioxide caused by human activity, keeping the Earth’s surface cooler than it would have been otherwise.

Are we getting close to passing 1.5 C?

Although some days this year have already surpassed the 1.5 degrees mark, the worst climate effects won’t occur unless there is long-term permanent shift in global temperatures, which is projected to occur in the early 2030s, according to the Intergovernmental Panel on Climate Change’s Sixth Assessment Synthesis, which was released earlier this year.

During the first nine months of 2023, the daily global average temperature exceeded 1.5 C on 86 days, according to the U.N. emissions report. By adding October and the first two weeks of November, that figure reaches 127 days, according to Copernicus, the European climate service.

What happens if we breach 1.5 C?

While 1.5 C seems like a small number, it represents a significant increase in accumulated heat across the planet. This extra heat is responsible for record temperatures and flooding across parts of the U.S., Europe and Asia this past summer. It has also contributed to marine heat waves that have bleached coral reefs, reduced polar snow cover and sea ice, and altered growing seasons that affect farmers worldwide.

The rise in atmospheric greenhouse gases that contribute to climate change is faster than at any time now than in the past several million years, according to Peter Jacobs, a climate scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md. At the same time, Jacobs said it isn’t too late for society to reverse course by cutting emissions.

“The first year that we go above 1.5 C, it isn’t like there’s no going back," said Jacobs. “You could conceivably warm past 1.5, reduce emissions to zero, and then draw down carbon dioxide somehow, cool off, and maybe still avoid some of those really bad impacts."

Write to Eric Niiler at eric.niiler@wsj.com

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