But We Knew That. Warming Unique in 2000 Years.

July 25, 2019

Calling Captain Obvious.

Smithsonian:

A group of climate scientists has reached a surprising conclusion about Earth’s past eras of naturally-driven, global warming and cooling—they weren’t global after all.

The authors of new studies in Nature and Nature Geoscience used evidence of ancient climates gathered around the world, from tree rings to coral reefs, to examine the pace and extent of well-known episodes of warming or cooling over the past 2,000 years. They report that events like the Little Ice Age and Mediaeval Warm Period, driven by natural variability, were actually more regional than global in scope.

In fact, the only time in the past 2,000 years that nearly all of the Earth has undergone significant warming or cooling is the present period of change that began in the 20th century, according to the research of Nathan Steiger, an atmospheric scientist at the Lamont-Doherty Earth Observatory of Columbia University, and colleagues. The rate of warming was also higher during the second half of the 20th century than during any similar period of the past 2,000 years, the studies found.

“It was surprising to us that the coherence of the climate, prior to the industrial revolution, was much more regional,” Steiger says. “There were regional periods of cold or warmth, but it’s only during the contemporary period where there’s a global warm period that’s very different from what we see in the past. On one hand it isn’t all that surprising that the climate now is fundamentally different, but this provides a really nice long-term context where can clearly see that contrast.”

Science News:

That finding stands in stark contrast to the team’s conclusions about the current era of warming: It is occurring concurrently around the globe, with the hottest temperatures in the study found to be right at the end of the 20th century. “It is coherent in a way we didn’t experience over the last 2,000 years,” Steiger says.

Because the study’s temperature data go only to 2000, the last two decades of temperatures weren’t included. But NASA and the National Oceanic and Atmospheric Administration reported in February that nine of the 10 warmest years on record have occurred since 2005, and the last five years were the five hottest on record (SN Online: 2/6/19). Human activities have been repeatedly cited by scientists as the cause of these ongoing record-breaking temperatures (SN Online: 7/2/19).

At a news conference on July 22, Steiger noted that the Nature study didn’t specifically mention that the current warming is due to anthropogenic activities, in part because so many previous studies have repeatedly and clearly demonstrated that link. “We don’t need to look at paleoclimate to know that.” (see video below)

A second study, published in Nature Geoscience, does address the question of anthropogenic warming more explicitly. The study, authored by Neukom and other members of the PAGES 2k Consortium, used the same temperature proxies as the Nature study. But instead of comparing spatial patterns of warming and cooling around the globe, the team looked at the average global temperature through time.

That, Neukom said at the news conference, revealed that the current rate of warming is much faster than anything observed in the last 2,000 years that can be attributed to natural variability. “It’s another angle to look at the extraordinary nature of current warming,” he said.

A third study, also in Nature Geoscience, added another layer of context to the trends, by looking at what natural forces may have been behind large regional temperature fluctuations such as the Little Ice Age.

Led by University of Bern climate scientist Stefan Brönnimann, the researchers found that before the onset of the Industrial Revolution in the mid-1800s, powerful volcanic eruptions were the main enginebehind such large-scale temperature fluctuations. For instance, five powerful eruptions, including the 1815 eruption of Mount Tambora, occurred toward the end of the Little Ice Age, which began around 1350 and ended around 1850 (SN: 7/7/18, p. 32).

The eruptions initially led to cooling and climate upheaval, and then a period of recovery as the planet warmed up again. That recovery coincides with the onset of the Industrial Revolution, at which point greenhouse gases became the primary driver for warming, the researchers note.

Mann notes that the studies’ findings — that these past climate epochs weren’t global and that current warming is proceeding at an unprecedented rate — aren’t exactly new ideas. In 1998, he and colleagues published a famous study in Nature, sometimes referred to as the “hockey stick” paper, that revealed a dramatic upward tick in temperatures at the end of the 20th century — a pattern that, when plotted through time, takes the shape of a hockey stick. “It’s gratifying that independent, international teams using entirely different approaches have come to virtually identical conclusions,” Mann says.

Raymond Bradley, a climate scientist at the University of Massachusetts Amherst who coauthored the hockey stick study, agrees. “In that paper, we were widely criticized for saying the last decade was the warmest in the last 1,000 years,” he says.

Bradley adds that the new studies are a valuable addition, with a “high quality, carefully screened set of data” as well as powerful simulations that can synthesize and reproduce those past temperatures. “They’ve done everything right,” he adds.

But the PAGES 2k database of temperature proxies still contains some glaring holes, particularly in tropical regions and in the oceans. Such data won’t change the underlying storyline, but could help scientists see global temperature fluctuations even more clearly, Bradley says. Efforts to collect more temperature data from South America are already afoot, from cave formations across Brazil and Argentina as well as from ancient trees in the Amazon forest, he adds, and “that’s exactly what’s needed.”

5 Responses to “But We Knew That. Warming Unique in 2000 Years.”

  1. redskylite Says:

    “But We Knew That. Warming Unique in 2000 Years.”

    Well I did know that from taking an online Penn State course, given by the esteemed Richard Alley nearly ten years ago, also a glance at the famous “hockey stick” makes it instantly obvious.

    But many people have no interest in the science or facts – they live busy, insulated lives, and don’t pay much attention or care to the existential crisis.

    So if studies periodically reinforce these known facts and it gets picked up by the media centers – the news may just sink in. I hope so, it our main chance.

    “Climate change: Current warming ‘unparalleled’ in 2,000 years”

    https://www.bbc.com/news/science-environment-49086783

  2. neilrieck Says:

    Dont’ forget this one titled:
    “Climate change: 12 years to save the planet? Make that 18 months”
    https://www.bbc.com/news/science-environment-48964736

  3. doldrom Says:

    Yesterday I saw Sander Houweling being interviewed on the quad at the university here, and when I looked at the broadcast on the national news later, I noted that he was talking about this very study (Steiger, Globally coherent warm and cold periods). Not everyday that a study mentioned on this blog rates national news coverage the next day. I found it somewhat gratifying, because whenever I hear mention of the Medieval Warm Period I already tense up for some gratuitous bogus denialism).

    Occasion for the attention was the record-breaking heat wave here. On Wednesday the record from 1944 (38.6) was marked up to 39.3, and on Thursday to 40.7/105.3°, so that’s a single increment of 2.1/3.8°. The official records go back to 1901, and the temperature series goes back to 1706 (the Netherlands; we’re at 52° latitude N here!). Hundreds of local records were reset. Record temperatures were also recorded all over Germany, Belgium, and France (Paris 42.6°/108.7F).


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