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Pacific Ocean Warming at Fastest Rate in 10,000 Years
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".
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is not as high as during Medieval times, i.e. during what they term the "Medieval Warm Period" (this is a somewhat outdated term; the term "Medieval Climate Anomaly" is generally favored by climate scientists because of the regionally variable pattern of surface temperatures changes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.
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Tensions are continuing to rise in Canada over a controversial pipeline project as protesters enter their 12th day blockading railways, demonstrating on streets and highways, and paralyzing the nation's rail system
Colorado River Has Lost 1.5 Billion Tons of Water to the Climate Crisis, 'Severe Water Shortages' May Follow
California is headed toward drought conditions as February, typically the state's wettest month, passes without a drop of rain. The lack of rainfall could lead to early fire conditions. With no rain predicted for the next week, it looks as if this month will be only the second time in 170 years that San Francisco has not had a drop of rain in February, according to The Weather Channel.
The last time San Francisco did not record a drop of rain in February was in 1864 as the Civil War raged.
"This hasn't happened in 150 years or more," said Daniel Swain, a climate scientist at UCLA's Institute of the Environment and Sustainability to The Guardian. "There have even been a couple [of] wildfires – which is definitely not something you typically hear about in the middle of winter."
While the Pacific Northwest has flooded from heavy rains, the southern part of the West Coast has seen one storm after another pass by. Last week, the U.S. Drought Monitor said more Californians are in drought conditions than at any time during 2019, as The Weather Channel reported.
The dry winter has included areas that have seen devastating fires recently, including Sonoma, Napa, Lake and Mendocino counties. If the dry conditions continue, those areas will once again have dangerously high fire conditions, according to The Mercury News.
"Given what we've seen so far this year and the forecast for the next few weeks, I do think it's pretty likely we'll end up in some degree of drought by this summer," said Swain, as The Mercury News reported.
Another alarming sign of an impending drought is the decreased snowpack in the Sierra Nevada Mountain range. The National Weather Service posted to Twitter a side-by-side comparison of snowpack from February 2019 and from this year, illustrating the puny snowpack this year. The snow accumulated in the Sierra Nevadas provides water to roughly 30 percent of the state, according to NBC Los Angeles.
Right now, the snowpack is at 53 percent of its normal volume after two warm and dry months to start the year. It is a remarkable decline, considering that the snowpack started 2020 at 90 percent of its historical average, as The Guardian reported.
"Those numbers are going to continue to go down," said Swain. "I would guess that the 1 March number is going to be less than 50 percent."
The National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center forecast that the drier-than-average conditions may last through April.
NOAA said Northern California will continue deeper into drought through the end of April, citing that the "persistent high pressure over the North Pacific Ocean is expected to continue, diverting storm systems to the north and south and away from California and parts of the Southwest," as The Weather Channel reported.
As the climate crisis escalates and the world continues to heat up, California should expect to see water drawn out of its ecosystem, making the state warmer and drier. Increased heat will lead to further loss of snow, both as less falls and as more of it melts quickly, according to The Guardian.
"We aren't going to necessarily see less rain, it's just that that rain goes less far. That's a future where the flood risk extends, with bigger wetter storms in a warming world," said Swain, as The Guardian reported.
The Guardian noted that while California's reservoirs are currently near capacity, the more immediate impact of the warm, dry winter will be how it raises the fire danger as trees and grasslands dry out.
"The plants and the forests don't benefit from the water storage reservoirs," said Swain, as The Mercury News reported. "If conditions remain very dry heading into summer, the landscape and vegetation is definitely going to feel it this year. From a wildfire perspective, the dry years do tend to be the bad fire years, especially in Northern California."
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A warm day in winter used to be a rare and uplifting relief.
Now such days are routine reminders of climate change – all the more foreboding when they coincide with news stories about unprecedented wildfires, record-breaking "rain bombs," or the accelerated melting of polar ice sheets.
Where, then, can one turn for hope in these dark months of the year?