Quantcast

Pacific Ocean Warming at Fastest Rate in 10,000 Years

Climate

Just how rapid is the current rate of warming of the ocean? There is an interesting new article by Rosenthal and collaborators in the latest issue of the journal Science, "Pacific Ocean Heat Content During the Past 10,000 Years" that attempts to address this question. The article compares current rates of ocean warming with long-term paleoclimatic evidence from ocean sediments. So how rapid is the ocean warming? Well, for the Pacific ocean at least, faster than any other time in at least the past 10,000 years.

The study finds, specifically, that (to quote Columbia University's press release) the "middle depths [of the Pacific Ocean] have warmed 15 times faster in the last 60 years than they did during apparent natural warming cycles in the previous 10,000".

How rapid is the ocean warming? For the Pacific ocean at least, faster than any other time in at least the past 10,000 years.

Beyond that key overall take-home conclusion, though, there are some enigmatic aspects of the study. The authors argue for substantial differences between proxy reconstructions of surface temperature and their new sediment core evidence of intermediate water temperatures from the tropical IndoPacific, during the past two millenia. The researchers argue that recent warmth is anomalous in the former case, but not the latter. They argue that, while the present rate of ocean warming is unprecedented, the actual level of ocean heat content (which depends not just on surface temperature, but also sub-surface ocean temperatures) is not as high as during Medieval times, i.e. during what they term the "Medieval Warm Period" (this is a somewhat outdated term; the term "Medieval Climate Anomaly" is generally favored by climate scientists because of the regionally variable pattern of surface temperatures changes in past centuries—more on this later).

One complication with their comparison is that the dramatic warming of the past half century is not evident in the various sediment data analyzed in the study. "Modern" conditions conditions are typically defined by the "tops" of the sediment core obtained by drilling down below the ocean bottom. But sediment core tops are notoriously bad estimates of "current" climate conditions because of various factors, including the limited temporal resolution owing to slow sediment deposition rates, and processes that mix and smear information at the top of the core. Core tops for these reasons tend not to record the most recent climate changes. Thus, the researchers' data do not explicitly resolve the large recent increases in temperature (and heat content).

But if the warming of the past half century is not resolved by their data, then the assumption that those data can be registered against a common modern baseline (the authors use a reference period of 1965-1970) too is suspect. That registration is critical to their conclusion that modern heat content has not exceeded the bounds of the past two millennia.

There are also some puzzling inconsistencies between the authors' current conclusions and other previously published evidence implying a very different pattern of global ocean heat content changes over the past two millennia. Current global sea level has been shown to be unprecedented for at least the past two millennia in previous work using both proxy-based sea level reconstructions and predictions from "semi-empirical" models of sea level change.

Thermal expansion due to sub-surface ocean warming is a substantial contributor to the observed rise this century in global sea level. It is thus difficult to reconcile the observation that modern sea level is unprecedented over at least the past two millennia with the authors' claim that there has not been an anomalous increase in global ocean heat content over this time frame. Given that there is unlikely to have been any sea level rise contribution from melting ice sheets prior to the most recent decades, any explanation would have to involve extremely large sea level contributions from the melting of small glaciers and ice caps, contributions that exceed what is actually evident in the climate record.

Finally, we need to maintain a healthy skepticism about broad conclusions about global climate drawn from one specific region like the tropical IndoPacific. It is surprising in this context that the article didn't mention or cite two studies published in the same journal (Science), a few years ago: Mann et al (2009) and Trouet et al (2009) which demonstrate a high degree of regional heterogeneity in global temperature changes over the past millennium. Both studies attribute much of that heterogeneity to dynamical climate responses related to the El Niño phenomenon. The tropical Pacific appears to have been in an anomalous La Niña-like state during the Medieval era. During such a state, which is the flip-side of El Niño, much of the tropical Pacific (the eastern and central tropical Pacific) is unusually cold. But the tropical western Pacific and IndoPacific are especially warm. That makes it perilous to draw inferences about global-scale warmth from this region (see this more detailed discussion at RealClimate).

There a few other minor, odd things about the study. In a figure comparing the sediment records with proxy reconstructions of surface temperature, the authors attribute one of the curves to "Mann 2003" in the figure legend. This would appear to be a reference to a rather old reconstruction by Mann and Jones (2003), which is supplanted by a newer, far more comprehensive study by Mann et al (2008). The authors indeed cite this latter study in footnote of the figure caption. So it is unclear which reconstruction is actually being shown, and the comparison is potentially inappropriate.

The authors, in a different figure, show a recent, longer albeit somewhat more tenuous reconstruction of global temperature over the past 11,000 years by Marcott et al (2013), published in Science earlier this year. That reconstruction was observed to be consistent with that of Mann et al (2008) during the interval of overlap of the past two millennia.

It is also puzzling that the article doesn't show or even cite the most comprehensive hemispheric reconstruction to date, that of the PAGES 2K Consortium published in the journal Nature Geoscience two months before the present paper was submitted to Science. That reconstruction demonstrates modern warming to considerably exceed the peak warmth of the Medieval period, closely resembling the original Mann et al "Hockey Stick." It would have been useful to see all of these reconstructions, each of which demonstrate recent warmth to be anomalous in a long-term context, compared on the same graph against the sediment series of this study.

In summary, the Rosenthal study is interesting and it provides useful new paleoclimate data that give us an incrementally richer understanding of the details of climate changes in pre-historic times. However, there are a number of inconsistencies with other evidence, and debatable assumptions and interpretations, which will require sorting out by the scientific community. That is, of course, the "self-correcting" machinery of science that Carl Sagan spoke so eloquently of.

Visit EcoWatch’s CLIMATE CHANGE page for more related news on this topic.

———

EcoWatch Daily Newsletter

Natural Resources Defense Council

By Emily Deanne

Shower shoes? Check. Extra-long sheets? Yep. Energy efficiency checklist? No worries — we've got you covered there. If you're one of the nation's 12.1 million full-time undergraduate college students, you no doubt have a lot to keep in mind as you head off to school. If you're reading this, climate change is probably one of them, and with one-third of students choosing to live on campus, dorm life can have a big impact on the health of our planet. In fact, the annual energy use of one typical dormitory room can generate as much greenhouse gas pollution as the tailpipe emissions of a car driven more than 156,000 miles.

Read More Show Less
Kokia drynarioides, commonly known as Hawaiian tree cotton, is a critically endangered species of flowering plant that is endemic to the Big Island of Hawaii. David Eickhoff / Wikipedia

By Lorraine Chow

Kokia drynarioides is a small but significant flowering tree endemic to Hawaii's dry forests. Native Hawaiians used its large, scarlet flowers to make lei. Its sap was used as dye for ropes and nets. Its bark was used medicinally to treat thrush.

Read More Show Less
Sponsored
Frederick Bass / Getty Images

States that invest heavily in renewable energy will generate billions of dollars in health benefits in the next decade instead of spending billions to take care of people getting sick from air pollution caused by burning fossil fuels, according to a new study from MIT and reported on by The Verge.

Read More Show Less
Aerial view of lava flows from the eruption of volcano Kilauea on Hawaii, May 2018. Frizi / iStock / Getty Images

Hawaii's Kilauea volcano could be gearing up for an eruption after a pond of water was discovered inside its summit crater for the first time in recorded history, according to the AP.

Read More Show Less
A couple works in their organic garden. kupicoo / E+ / Getty Images

By Kristin Ohlson

From where I stand inside the South Dakota cornfield I was visiting with entomologist and former USDA scientist Jonathan Lundgren, all the human-inflicted traumas to Earth seem far away. It isn't just that the corn is as high as an elephant's eye — are people singing that song again? — but that the field burgeons and buzzes and chirps with all sorts of other life, too.

Read More Show Less
Sponsored
A competitor in action during the Drambuie World Ice Golf Championships in Uummannaq, Greenland on April 9, 2001. Michael Steele / Allsport / Getty Images

Greenland is open for business, but it's not for sale, Greenland's foreign minister Ane Lone Bagger told Reuters after hearing that President Donald Trump asked his advisers about the feasibility of buying the world's largest island.

Read More Show Less
AFP / Getty Images / S. Platt

Humanity faced its hottest month in at least 140 years in July, the US National Oceanic and Atmospheric Administration (NOAA) said on Thursday. The finding confirms similar analysis provided by its EU counterparts.

Read More Show Less
Newly established oil palm plantation in Central Kalimantan, Indonesia. Rhett A. Butler / Mongabay

By Hans Nicholas Jong

Indonesia's president has made permanent a temporary moratorium on forest-clearing permits for plantations and logging.

It's a policy the government says has proven effective in curtailing deforestation, but whose apparent gains have been criticized by environmental activists as mere "propaganda."

Read More Show Less