Searing statements in this week’s landmark Intergovernmental Panel on Climate Change (IPCC) report are particularly alarming, considering the characteristically cautious language of science. The first consensus of the document’s 234 authors: it is “unequivocal” that humanity’s burning of fossil fuels has caused climate change. In fact, data reconstructions in the report indicate that Earth’s average surface temperature has likely not been this warm over a long period in about 125,000 years [see “Change in Global Surface Temperature”].

The latest IPCC report pulls no punches in describing the consequences of rising greenhouse gas emissions, noted Kim Cobb, a climate scientist at the Georgia Institute of Technology and one of the assessment’s lead authors, at a prerelease briefing for journalists. With more certainty than past IPCC reports, Cobb and her co-authors conclude that climate change is already driving extreme heat waves, droughts, heavy rainfalls and tropical cyclones across the globe. In all possible emissions scenarios studied, temperatures will rise until at least 2050 and are likely to surpass 1.5 degrees Celsius by 2040. But if emissions are brought to net zero by 2050—meaning that any released greenhouse gases are balanced by the amounts removed from the atmosphere—warming can still be limited to 1.5 degrees C over the course of this century.

The nearly 4,000-page document, which cites more than 14,000 studies, is the IPCC’s first major assessment of the state of climate science since 2013. The report’s certainty is based not only on a better understanding of Earth’s climate system but also on investigations into its climatic past. Data from corals, tree rings, cores extracted from ice or marine sediments, and other sources allow paleoclimate researchers such as Cobb to peer beyond modern records and into ancient worlds, generating crucial insights for climate science and forecasts today. Scientific American spoke with Cobb about the role of paleoclimate data in refining the IPCC’s new report and the importance of providing context for Earth’s current warming.

[An edited transcript of the interview follows.]

Why is it important to look at our climatic past?

Given that climate records from instrumental measurements often only go back 50 years—sometimes maybe 100 or 150—we cannot really capture the full breadth of natural variability in Earth’s climate system. What we can do with paleoclimate archives is extend that record back in time. Now we have enough records across many sites in the world to make large-scale averages—the kind that allow us to directly make comparisons with what’s going on today across the planet. It’s important to provide that context for just how quickly things are changing today and how unusual this is.

How was paleoclimate research put to use in this new report?

Paleoclimate research has been part of every IPCC assessment since the first one in 1990. The difference here is that rather than being relegated to its own chapter, there’s an attempt to integrate these lines of evidence into every aspect of the report. Climate scientists across every discipline now understand the richness of information available from paleoclimate sources. The field has matured in recent decades to deliver quantitative information that lends itself to the types of quantitative analyses that are most common in studies of current climate change.

Do any precedents exist in the geologic record for the climatic changes we are seeing today?

It’s interesting to look at a time around 125,000 years ago, during the last interglacial period. Our best estimates of temperature increases during that interval are about one to two degrees Celsius—not unlike where we’re parked today, at roughly one degree C warmer than preindustrial times. Back then, the increases were driven by changes in Earth’s orbit with respect to the sun, and the warming was sustained for long enough over many thousands of years to elicit melting of the Greenland ice sheet, driving global sea levels five to 10 meters higher. These numbers are big exclamation marks on where our planet has been in the not-too-distant geologic past and on where we’re headed in the long term as Earth fully responds to the levels of warming we’re already at.

None of Earth’s past warm periods is an appropriate analogue for what we’re seeing today, however. The rates of what we’re undertaking right now tend to distinguish current climate change from past changes of this magnitude that have happened over much longer timescales and are caused by natural climate drivers.

 Graph shows change in global surface temperature as reconstructed from A.D. 1 to 2000 and observed from 1850 to 2020.
Credit: Amanda Montañez; Source: Climate Change 2021: The Physical Science Basis: Summary for Policymakers. Working Group 1 to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press (in press)

This report has new estimates for an important metric called climate sensitivity. What is that, and how have paleoclimate studies helped in estimating it?

Climate sensitivity is essentially how much warming occurs per unit of carbon dioxide increase in the atmosphere. Higher climate sensitivity means more warming for a given CO2 increase, and vice versa. It’s one of the uncertainties of future emissions impacts, but this report has narrowed its range significantly, relative to past reports.

Paleoclimate science can contribute to this conversation around climate sensitivity by looking into the temperature response to past changes in CO2, grounded in data describing the planet’s past climatic conditions.

For instance, my colleague Jessica Tierney at the University of Arizona has worked hard to compile a global database of temperatures from the last glacial period, around 20,000 years ago, and used it to come up with estimates for global cooling during that time. We know CO2 very well, so with those two numbers, she and her co-authors were able to calculate estimates for climate sensitivity.

The report forecasts that even if we succeed in limiting warming to 1.5 degrees C, sea levels will continue to rise through 2050. Why is that?

This heating we’ve baked into the atmosphere to date has already permeated into the interior of the ice sheets. The adjustment to the excess heat that we’ve caused—and will yet cause—is going to be imprinted into the continued slow melting of these massive ice sheets for centuries and maybe millennia to come.

The good news is that if we do enact deep and sustained reductions in greenhouse gas emissions right now, we’ll see the benefits of that later this century in the form of reduced rates or magnitudes of sea-level rise. Where do we want the sea level to land? And do we want to grant as much time as possible to future generations to adapt? Those are questions we face this decade and the next.

Are any other processes now essentially “locked in” for long timescales?

In general, ocean impacts are going to take longer to reverse. One important impact is ocean acidification—the signature of stored carbon in the ocean. We’ve already altered the chemistry of the upper ocean down to 2,000 meters—almost half its depth in some places. The carbon won’t magically come out all at once; it will come out as these high-CO2 waters are exposed at the surface to an atmosphere lower in CO2. We have to wait for that exchange to happen because deep waters must be brought to the surface again. It will be a relatively slow process.

That doesn’t conflict with one of the core messages of the report, which is that when we reach net zero emissions, we’ll begin to see an almost immediate and discernible stabilization—if not a reversal—in global warming itself. Many impacts that are directly related to global surface temperatures, such as heat waves, might be some of the earliest to respond.

How might paleoclimate research continue to add to our understanding of climate change?

Peering back into the paleoclimate record can help us understand how groups of extremes or cycles have evolved in the recent past and how they might be changing in response to climate change. It’s becoming increasingly apparent that these approaches are extremely relevant for our climate future. We can turn to archives of past extremes—whether it’s droughts, extreme rainfall, tropical cyclones—or natural climate cycles to understand more about their natural variability over the past centuries to millennia. One example is El Niño events, the warm phases of a natural climate cycle that occurs in the tropical Pacific Ocean. In the past 70 years, we’ve really only seen a handful—10, give or take. But by looking into the paleoclimate archive of El Niño events, we can analyze hundreds over the past millennia. Just last year, we published a paper documenting an increase in El Niño intensity over recent decades, compared with the preindustrial period.

Are there insights that we can draw on for climate optimism these days?

I’m somewhat relieved to see that the rest of the world can come to a level of awareness that I’ve had over the past five to eight years. When you have to keep up with the science for your day job, you digest it in real time. Now it feels like a little bit of weight has lifted—that at least the rest of the world knows what I do at this moment. That’s a huge relief.

I hope that new awareness will spur the kind of action needed to walk that path of deep, sustained emissions reductions that will limit warming and reserve the right to cool later this century. That’s what I’m fighting for.