IPCC Sixth Assessment Report

Based on Wikipedia: IPCC Sixth Assessment Report

Here's a number that should keep you up at night: 500 billion tonnes. That's roughly how much more carbon dioxide humanity can pump into the atmosphere before we've essentially flipped a coin on whether we'll blow past 1.5 degrees Celsius of warming. At current emission rates, we'll burn through that budget in about a decade.

This isn't speculation from environmental activists or doomsday prophets. It's the conclusion of 234 scientists from 66 countries, drawing on more than 14,000 scientific papers, approved line-by-line by 195 governments—including major oil producers. The document is called the Sixth Assessment Report, published by the Intergovernmental Panel on Climate Change (commonly known as the IPCC), and it represents the most comprehensive scientific consensus on climate change ever assembled.

What the IPCC Actually Is

The IPCC doesn't conduct new research. Instead, it synthesizes existing scientific literature—thousands of peer-reviewed studies—into a coherent picture of what we know about climate change. Think of it as the world's largest scientific book club, where the members happen to be leading climate researchers, and the reading list is every relevant study published since the last report.

What makes these reports unusual is the approval process. Every sentence in the "Summary for Policymakers"—the executive summary that world leaders actually read—must be agreed upon by government representatives from nearly every nation on Earth. This isn't the United Nations forcing conclusions on scientists; it's scientists presenting conclusions that governments cannot credibly deny. When Saudi Arabia and the Marshall Islands both sign off on the same document about fossil fuel emissions, you're witnessing something remarkable.

The Sixth Assessment came out in stages between 2021 and 2023, divided into three working groups. The first tackled the physical science—the basic question of what's happening to the climate and why. The second examined impacts, asking what climate change will do to ecosystems and human societies. The third focused on mitigation, exploring what we can do about it. A final synthesis report tied everything together in March 2023, just in time to inform the COP28 climate summit in Dubai.

The Hockey Stick Gets Longer

You may have heard of the "hockey stick" graph—that famous chart showing relatively stable global temperatures for centuries, then a sharp upward spike in recent decades. The Sixth Assessment extended this graph with additional historical data, and the blade of the hockey stick has only grown more dramatic.

The report introduced something new: for the first time, geopolitics was incorporated into climate models. Scientists developed five "Shared Socioeconomic Pathways"—essentially different stories about how the 21st century might unfold, each with its own assumptions about population growth, economic development, international cooperation, and technology.

These pathways have evocative names. "Taking the Green Road" imagines a world of sustainable development and international cooperation. "Middle of the Road" continues current trends. "A Rocky Road" envisions nationalism and regional conflicts undermining climate action. "A Road Divided" describes a world of stark inequality. And "Taking the Highway" represents aggressive fossil fuel development with the hope that technology will eventually save us.

The difference between these futures is staggering. Under the most optimistic pathway, global temperatures might peak around 1.6 degrees Celsius above pre-industrial levels around mid-century, then gradually decline. Under the worst scenarios, we could see 5 degrees Celsius of warming—a world that would be almost unrecognizable.

What "Climate Sensitivity" Actually Means

Scientists have a concept called "climate sensitivity"—it measures how much global temperature rises when you double the amount of carbon dioxide in the atmosphere. The Sixth Assessment narrowed this estimate to between 2.5 and 4 degrees Celsius, with 3 degrees as the best guess.

To put that in perspective: pre-industrial carbon dioxide levels were about 280 parts per million. We've already pushed past 420 parts per million. We haven't quite doubled it yet, but we're well on our way, and the carbon already in the atmosphere will continue warming the planet for decades.

This sensitivity range is tighter than previous estimates, which is actually bad news. Earlier reports allowed for the possibility that the climate might be less sensitive to carbon dioxide—that we might get lucky. The new research closes off that escape route. We're almost certainly looking at significant warming even under moderate emission scenarios.

The Math of Extreme Weather

Perhaps the most striking element of the report is a table showing how extreme weather events multiply with temperature. Consider a heat wave that used to occur once every 50 years—the kind of scorching week that older residents remember as exceptional.

At 1.5 degrees of warming, that event becomes 8.6 times more likely. At 2 degrees, it's 13.9 times more likely. At 4 degrees, it happens 39.2 times more often. What was once a twice-in-a-lifetime experience becomes something you expect most summers.

The intensity increases too. That once-in-50-years heat wave gets about 2 degrees hotter at 1.5 degrees of global warming, 2.7 degrees hotter at 2 degrees of warming, and 5.1 degrees hotter at 4 degrees. The difference between a dangerous heat wave and a lethal one often comes down to just a few degrees.

Heavy rainfall events follow a similar pattern. A once-in-10-years downpour becomes 1.5 times more likely at 1.5 degrees of warming, and 2.7 times more likely at 4 degrees. The extra heat in the atmosphere holds more moisture, which has to come down somewhere.

One breakthrough in the Sixth Assessment is the science of "attribution"—linking specific weather events to climate change. Previous reports could only speak in generalities about trends. Now scientists can examine an individual heat wave or hurricane and calculate how much more likely climate change made it. This transforms abstract statistics into concrete accountability.

Compound Effects: When Bad Things Happen Together

The report pays unusual attention to "compound events"—when multiple climate impacts hit simultaneously. A drought by itself is manageable. A heat wave by itself is survivable. A drought plus a heat wave creates conditions where crops fail, forests burn, and people die.

These compound effects were rated more seriously than in any previous IPCC report. Climate systems interact in ways that simple models miss. Rising seas make storm surges more destructive. Melting permafrost releases methane, accelerating warming. Dying forests stop absorbing carbon, pushing temperatures higher still.

Sea level rise offers a particularly sobering example. The report estimates between half a meter and one meter of rise by 2100 under most scenarios—enough to threaten coastal cities worldwide. But it also notes that two to five meters cannot be ruled out, because the behavior of ice sheets at higher temperatures remains poorly understood. The Greenland and Antarctic ice sheets contain enough frozen water to raise global sea levels by about 70 meters. We don't fully understand what will make them collapse, or how fast it might happen.

Who Gets Hurt

The second working group's report landed during a particularly dramatic week in February 2022—the first week of Russia's invasion of Ukraine. In a moment of unusual frankness, a Russian scientist on the panel publicly apologized for his country's aggression, despite potential consequences back home. The Ukrainian delegation drew a direct line between the war and global fossil fuel dependence, and pleaded for climate coverage not to be drowned out by conflict reporting.

Their concerns were well-founded. The report contained devastating findings about who would suffer most from climate change, and the answer is: almost everyone, but especially the poor.

At least 3.3 billion people—roughly 40 percent of humanity—now live in conditions classified as "highly vulnerable" to climate impacts. Africa faces the loss of 30 percent of its maize-growing territory and half of its bean-cultivation land. One billion people face flooding from sea level rise. The report identified 127 distinct negative impacts of climate change, some of them irreversible.

Wealthier nations aren't immune. The report cites evidence that China will face the highest absolute financial costs as temperatures rise, including food and water insecurity, coastal flooding, and increasingly powerful cyclones. Parts of the country may eventually experience "wet-bulb temperatures" that humans physically cannot survive for more than six hours. Wet-bulb temperature combines heat and humidity—when it exceeds about 35 degrees Celsius, the human body can no longer cool itself through sweating, no matter how healthy or well-hydrated you are.

The Limits of Adaptation

The report introduces a concept called "adaptation limits"—the point beyond which people can no longer adjust to changing conditions. Move to higher ground to escape flooding, and you can adapt. But when the flooding reaches higher ground too, you've hit a limit. Switch to drought-resistant crops, and you can adapt. But when temperatures exceed what any crops can tolerate, adaptation fails.

Some adaptation is happening. Water management has improved in many regions. Urban agriculture is expanding. Conservation efforts are protecting crucial ecosystems. The report notes that effective protection requires preserving 30 to 50 percent of Earth's land, freshwater, and ocean areas—an ambitious target, but one based on the biological reality of what ecosystems need to survive.

But adaptation has limits, and the report is skeptical of technological silver bullets. Carbon capture technologies remain expensive and unproven at scale. Solar radiation management—spraying particles into the atmosphere to reflect sunlight—carries risks that aren't well understood. Planting forests in inappropriate locations can actually make things worse. Bioenergy projects, if poorly implemented, may cause more problems than they solve.

The report is particularly pointed about "loss and damage"—the harm that cannot be prevented through adaptation. This is a politically sensitive concept because it implies responsibility. If developed nations caused most historical emissions, and developing nations suffer most of the consequences, who should pay? Rich countries have historically resisted this framing, but the science increasingly forces the question.

The Overshoot Problem

Here's an uncomfortable truth buried in the report: under every scenario the scientists modeled, global temperatures exceed 1.5 degrees Celsius at least temporarily. Even the most aggressive emission cuts can't prevent that threshold from being crossed at some point in the coming decades.

This has led to discussion of "overshoot"—the idea that we might temporarily exceed temperature targets and then bring warming back down through carbon removal. The report is cautious about this approach. Some climate impacts are irreversible. Species that go extinct don't come back. Permafrost that melts releases greenhouse gases that accelerate warming regardless of what we do afterward. An overshoot isn't a reset button.

The report states bluntly that climate-resilient development becomes "impossible in some regions and sub-regions" if warming exceeds 2 degrees Celsius. Not difficult. Impossible. Some places will become effectively uninhabitable—too hot, too flooded, too storm-battered for human societies to function.

The Methane Window

One finding offers a sliver of tactical hope: methane matters enormously in the short term. Unlike carbon dioxide, which persists in the atmosphere for centuries, methane breaks down within about a decade. This means cutting methane emissions produces rapid results.

The report emphasizes that rapidly reducing methane is critical for buying time while longer-term carbon dioxide reductions take effect. Major sources of methane include livestock, rice paddies, landfills, and—crucially—leaks from natural gas infrastructure. Unlike phasing out coal plants or redesigning transportation systems, many methane reductions could be achieved relatively quickly with existing technology.

What Would It Take

Joeri Rogelj, director of the Grantham Institute and a lead author on the report, laid out what avoiding 1.5 degrees of warming would actually require: cutting global emissions by 50 percent by 2030, and reaching 100 percent reduction—true net zero—by 2050.

Consider what that means. In 2030, seven years after the report's publication, worldwide greenhouse gas emissions would need to be half of current levels. Not half of projected growth. Half of today's emissions. That's roughly the equivalent of eliminating all emissions from the United States, Europe, and Japan combined.

By 2050, every remaining source of emissions would need to be offset by carbon removal. Every factory, every farm, every vehicle, every building—either converted to zero-emission alternatives or balanced by drawing carbon from the atmosphere.

Is this possible? The scientists who wrote the report believe so, with caveats. It requires immediate action. Every year of delay makes the targets harder to reach. If the world hasn't begun drastic emission cuts by the next IPCC report, they write, it will no longer be possible to prevent 1.5 degrees of warming at all.

Where We Actually Stand

In January 2025, climate scientist Zeke Hausfather—one of the researchers who helped frame the emission scenarios—published an updated estimate. Based on current policies and trends, warming by 2100 now appears likely to land between 2.9 and 3.7 degrees Celsius.

This is simultaneously good and bad news. It's better than the worst-case scenarios of 4 to 5 degrees that seemed plausible a decade ago. Renewable energy costs have fallen faster than expected. Electric vehicles are gaining market share. Many countries have strengthened their climate policies.

But 3 degrees of warming is still catastrophic by any reasonable measure. It means heat waves that kill. Coastlines that flood. Breadbaskets that fail. Migrations that overwhelm. Wars fought over water and arable land. Extinction of species we haven't even catalogued yet.

The Sixth Assessment Report didn't assign probabilities to its scenarios—it presented them as a range of possibilities. But subsequent analysis suggests we've likely avoided the very worst outcomes while remaining far from the path to safety. We're no longer driving toward the cliff at maximum speed, but we haven't turned the wheel yet either.

The Starkest Warning Yet

The Guardian described the report as the IPCC's "starkest warning yet" of "major inevitable and irreversible climate changes." That phrasing—inevitable and irreversible—deserves attention.

Inevitable means that some changes will happen regardless of what we do now. The carbon already emitted has committed us to decades of additional warming. Some ice will melt. Some seas will rise. Some species will disappear. The question is no longer whether climate change will affect us, but how much.

Irreversible means that some changes cannot be undone on any timescale meaningful to human civilization. Extinct species don't return. Collapsed ecosystems don't rebuild themselves. Melted ice sheets take millennia to reform. We are making permanent alterations to the only planet we have.

But the report also maintains that every fraction of a degree matters. The difference between 1.5 degrees and 2 degrees is measured in millions of lives, in species preserved or lost, in cities that survive or drown. The difference between 2 degrees and 3 degrees is larger still. We may have lost the chance to avoid all harm, but we retain the power to decide how much harm we accept.

The synthesis report, completed in March 2023, served as the scientific foundation for COP28 in Dubai—a climate summit held, with considerable irony, in one of the world's major oil-producing nations. The negotiations that followed were contentious, incremental, and by most accounts insufficient. But they proceeded from a scientific foundation that no credible government could dispute, because 195 governments had already signed off on every word.

That may be the report's lasting contribution: not changing minds, but closing off exits. When the world's scientists and the world's governments agree on the facts, the argument shifts from whether we have a problem to what we're willing to do about it. The Sixth Assessment Report doesn't answer that question. It just makes clear that we can no longer pretend we aren't asking it.