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Scientists said that atmospheric carbon dioxide concentrations peaked above 430 parts per million for the first time in perhaps 30 million years.
The concentration of carbon dioxide in the atmosphere peaked above 430 parts per million in 2025—the highest it has been in millions of years—according to data released by the National Oceanic and Atmospheric Administration and Scripps Institution of Oceanography at the University of California, San Diego on Thursday.
The news was overshadowed by the explosive feud between U.S. President Donald Trump and his erstwhile backer Elon Musk, but climate activist Bill McKibben argued that it was ultimately more consequential.
"In the long run, this is actually going to be the important news of the day—CO2 in the atmosphere passes another grim milestone," McKibben wrote on social media.
In the long run, this is actually going to be the important news of the day--co2 in the atmosphere passes another grim milestone
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— Bill McKibben (@billmckibben.bsky.social) June 5, 2025 at 4:33 PM
Carbon dioxide has been accumulating in the atmosphere due primarily to the human burning of fossil fuels, as well as by the clearing of forests and other natural carbon sinks. There, it acts as a greenhouse gas, trapping heat from the Earth, and is the primary gas responsible for the rise of global temperatures by approximately 1.1°C from the 1850 -1900 average. This warming has already had a host of dramatic impacts, from extreme weather events to sea-level rise to polar ice melt, and scientists warn these impacts will only accelerate under current energy policies, which put the world on track for around 3°C of warming by 2100.
The last time that atmospheric CO2 concentrations topped 430 ppm was most likely more than 30 million years ago, Ralph Keeling, who directs the Scripps CO2 Program, toldNBC News.
"It's changing so fast," he said. "If humans had evolved in such a high-CO2 world, there would probably be places where we wouldn't be living now. We probably could have adapted to such a world, but we built our society and a civilization around yesterday's climate."
"While largely symbolic, passing 430 ppm should be a wake-up call."
Scripps and National Oceanic and Atmospheric Administration both measure carbon dioxide levels from NOAA's Mauna Loa Observatory in Hawaii, where Charles Keeling began taking measurements in 1958. As CO2 levels rise over time, they also follow a seasonal cycle—peaking in May before falling in the Northern Hemisphere summer and rising again in the fall.
This May, Scripps Oceanography calculated an average of 430.2 ppm for 2025, which is 3.5 ppm over the average for May 2024. NOAA's Global Monitoring Laboratory, meanwhile, calculated a monthly average of 430.5 ppm, a 3.6 ppm jump from the year before and the second-steepest yearly climb since 1958.
"Another year, another record," Keeling said in a statement. "It's sad."
The news comes two months after Mauna Loa daily measurements surpassed 430 ppm for the first time in March, which Plymouth Marine Laboratory professor Helen Findlay called "extremely disappointing and worrying."
"While largely symbolic, passing 430 ppm should be a wake-up call, especially given the accelerated response we are seeing of glaciers and ice sheets to current warming," Dr. James Kirkham, chief scientist of the Ambition on Melting Ice coalition of governments, said at the time.
"This upward trajectory is a direct result of continued fossil fuel use, likely exacerbated by emissions from extreme wildfires last year, methane leaks from fossil fuel extraction and possibly greater permafrost emissions, alongside decreased ability of very warm oceans to absorb CO2," Kirkham said.
The monthly record also comes a little more than a week after a United Nations report warned that there was a small chance global temperatures could surpass 2°C in at least 1 of the next 5 years, only a decade after world leaders pledged in the Paris agreement to keep global temperatures "well-below" that level.
"Carbon emissions are still rising, and the atmosphere is going to keep heating further until greenhouse gas concentrations stabilize," Matt Kean, who chairs Australia's Climate Change Authority, wrote in response to the Scripps and NOAA figures. "What sort of climate do we want to leave our children and those who come after them?"
With governments "scaling back their already meager" actions to tackle climate breakdown, said one ecologist, "our present-day human culture is on a suicide course."
Less than six months away from the next United Nations summit for parties to the Paris climate agreement, scientists on Tuesday released a study showing that even meeting the deal's 1.5°C temperature target could lead to significant sea-level rise that drives seriously disruptive migration inland.
Governments that signed on to the 2015 treaty aim to take action to limit global temperature rise by 2100 to 1.5°C beyond preindustrial levels. Last year was not only the hottest in human history but also the first in which the average global temperature exceeded 1.5°C. Multiple studies have warned of major impacts from even temporarily overshooting the target, bolstering demands for policymakers to dramatically rein in planet-heating fossil fuels.
The study published Tuesday in the journal Nature Communications Earth and Environmentwarns that 1.5°C "is too high" and even the current 1.2°C, "if sustained, is likely to generate several meters of sea-level rise over the coming centuries, causing extensive loss and damage to coastal populations and challenging the implementation of adaptation measures."
"To avoid this requires a global mean temperature that is cooler than present and which we hypothesize to be closer to +1°C above preindustrial, possibly even lower, but further work is urgently required to more precisely determine a 'safe limit' for ice sheets," the paper states, referring to Antarctica and Greenland's continental glaciers.
Co-author Jonathan Bamber told journalists that "what we mean by safe limit is one which allows some level of adaptation, rather than catastrophic inland migration and forced migration, and the safe limit is roughly 1 centimeter a year of sea-level rise."
"If you get to that, then it becomes extremely challenging for any kind of adaptation, and you're going to see massive land migration on scales that we've never witnessed in modern civilization," said the University of Bristol professor.
In terms of timing, study lead author Chris Stokes, from the United Kingdom's Durham University, said in a statement that "rates of 1 centimeter per year are not out of the question within the lifetime of our young people."
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There are currently around 8.18 billion people on the planet. The study—funded by the United Kingdom's Natural Environment Research Council—says that "continued mass loss from ice sheets poses an existential threat to the world's coastal populations, with an estimated 1 billion people inhabiting land less than 10 meters above sea level and around 230 million living within 1 meter."
"Without adaptation, conservative estimates suggest that 20 centimeters of [sea-level rise] by 2050 would lead to average global flood losses of $1 trillion or more per year for the world's 136 largest coastal cities," says the study, also co-authored by University of Wisconsin-Madison professor Andrea Dutton and University of Massachusetts Amherst's Rob DeConto in the United States.
DeConto said Tuesday that "it is important to stress that these accelerating changes in the ice sheets and their contributions to sea level should be considered permanent on multigenerational timescales."
"Even if the Earth returns to its preindustrial temperature, it will still take hundreds to perhaps thousands of years for the ice sheets to recover," the professor explained. "If too much ice is lost, parts of these ice sheets may not recover until the Earth enters the next ice age. In other words, land lost to sea-level rise from melting ice sheets will be lost for a very, very long time. That's why it is so critical to limit warming in the first place."
Not really a surprise We are already committed to a 12m+ sea-level rise, sufficient to drown every coastal town and city on the planet The only question is how long will this take www.theguardian.com/environment/...
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— Prof Bill McGuire (@profbillmcguire.bsky.social) May 20, 2025 at 7:50 AM
While the paper sparked some international alarm, Stokes highlighted what he called "a reason for hope," which is that "we only have to go back to the early 1990s to find a time when the ice sheets looked far healthier."
"Global temperatures were around 1°C above preindustrial back then, and carbon dioxide concentrations were 350 parts per million, which others have suggested is a much safer limit for planet Earth," he said. "Carbon dioxide concentrations are currently around 424 parts per million and continue to increase."
The new paper continues an intense stream of bleak studies on the worsening climate emergency, and specifically, looming sea-level rise. Another, published by the journal Nature in February, shows that glaciers have lost an average of 273 billion metric tons of ice annually since 2000.
Despite scientists' warnings, the government whose country is responsible for the largest share of historical planet-heating emissions, the United States, is actually working to boost the fossil fuel industry. Upon returning to office in January, U.S. President Donald Trump declared an "energy emergency" and ditched the Paris agreement.
Responding to the new study on social media, Scottish ecologist Alan Watson Featherstone called out both the U.S. and U.K. governments. He said that with many countries "scaling back their already meager and [totally] inadequate actions to address climate breakdown, our present-day human culture is on a suicide course."
A dangerous initiative smuggles in a blatantly imperial and morally bankrupt agenda in a grotesque attempt to curry favor with a nationalist and climate-denying American right.
On the heels of a new United Nations report finding that over 150 “unprecedented” floods, heatwaves, hurricanes, and other climate disasters struck in 2024, the Council on Foreign Relations has launched its new “Climate Realism Initiative.” The Initiative’s goals proffer a dangerous and ahistorical set of climate politics, washing the United States’ hands of any responsibility to clean up global emissions or cooperate internationally to prevent the catastrophic impacts of 3°C of warming.
In a recent article branding the Initiative, CFR fellow Varun Sivaram shamelessly lays out the three main pillars of so-called “climate realism”: (1) that the world will overshoot the Paris agreement’s target to limit warming to 1.5 and even 2°C, (2) that the U.S. should eschew its own emissions reductions in favor of investing domestically in new clean technologies that can compete globally, and (3) that the U.S. should lead international efforts to avert catastrophic climate change.
In light of the first and second, the hypocrisy of CFR’s third pillar is particularly absurd.
CFR’s agenda is as tone-deaf as it is without bearing in history, science, or morality.
On the first pillar, Sivaram argues we should simply accept and prepare for a world with 3°C of warming—his so-called “realism”—but doesn’t stop to share what such a “real” world would look like.
At 3°C, 3.25 billion people will be exposed to lethal heat and humidity every year. The number of people globally who lack sufficient access to water will double. The majority of coral reefs will die. Sea levels will rise, threatening low-lying islands like the Marshall Islands in the Pacific and coastal cities like Bangkok, Shanghai, Amsterdam, and New Orleans. Agricultural yields will tumble, with most crops across the world suffering.
Perhaps most terrifying, the risk of hitting irreversible and catastrophic climate tipping points—like the wholesale dying off of the Amazon or melting of the Arctic—significantly increases.
Stepping back for a moment, it’s important to remember that the Paris agreement’s 1.5°C target came to be because frontline countries demanded such a target. With the Global North coalescing around 2°C ahead of COP21 in Paris and anything more ambitious thus thought politically infeasible, small island countries stunned many observers in leading more than 100 countries in demanding “1.5°C to stay alive.” Such a target, many, like the Marshall Islands’ Tina Stege, argue is necessary to avoid inundating and erasing island nations and low-lying places across the world.
Yet, rich countries in the Global North—and notably the U.S.—have too often ignored these calls in favor of a target that enables the continued extraction and burning of fossil fuels, prioritizes profits today over catastrophe tomorrow, and maintains the enormous wealth gap between Global North and South. By arguing that the U.S. should cast off the world’s 1.5°C and even 2°C target, Sivaram simultaneously condemns the Global South to a future with catastrophic and irreversible warming, a world without islands, where the Marshall Islands as we know them simply cease to exist.
It is within this context, then, that Sivaram advances the Initiative’s second pillar by presenting the following chart. With it, he argues that reducing U.S. emissions won’t make a meaningful difference because they’re a small share of projected future total global emissions.
However, in so doing, Sivaram ignores—even obfuscates—historical emissions. Consider a different chart, this one from Climate Watch, which illustrates the U.S.’ and the broader Global North’s role in creating the climate crisis in the first place. Looking back to the late 1800s, the U.S. and the European Union are responsible for over 50% of historical global greenhouse emissions (in CO2e).
In contrast, Small Island Developing States (SIDS)—a group of 39 island nations, including the Marshall Islands, across the Caribbean, Pacific, Atlantic, Indian Ocean, and South China Sea—have collectively contributed less than 1% of global emissions. Yet, SIDS and their nearly 65 million inhabitants are on the frontlines of the climate crisis, threatened by intensifying hurricanes and cyclones, shrinking biodiversity, and rising seas that threaten to swallow them whole.
Thus, Sivaram’s imperial assertion that U.S. emissions aren’t relevant to a “climate realism” agenda ignores what climate justice advocates have been raising for decades: that those most responsible for climate change should, in turn, be most responsible for addressing it. Instead, Sivaram offers an ahistorical perspective on emissions in service of uncapped emissions and U.S. exemption from climate accountability.
And then, finally, Sivaram offers his astoundingly contradictory final pillar: that the U.S. should lead efforts to avert catastrophic climate change. With the U.S. already a historic laggard and obstructionist in global climate negotiations, it’s hard to imagine a world in which the U.S. could possibly be seen to lead on climate while ignoring its own emissions reductions and sacrificing broad swaths of the Global South to sea-level rise, deadly heatwaves, and cascading crises driven by climate.
CFR’s agenda is as tone-deaf as it is without bearing in history, science, or morality. This dangerous initiative is anything but realistic, instead smuggling in a blatantly imperial and morally bankrupt agenda in a grotesque attempt to curry favor with a nationalist and climate-denying American right.
The climate movement must swiftly denounce this agenda and work toward one that aims to avoid overshoot at all costs, repair historic injustice, and uphold the value and dignity of human life across the globe.