A new study published Mar. 8 in the journal Science reports that global surface temperatures are now the hottest that they have been in the last 4,000 years, with the rate at which they are rising being the fastest seen in over 11,000 years. The study projects that temperatures may rise by as much as 11.3 degrees Fahrenheit (6.3 degrees Celsius) by the end of this century, well above the generally accepted 4 degrees Celsius that scientists previously agreed we were on track to reach without intervention.
Anthropogenic global warming, or changes in the earth's temperature caused by human activity, namely the greenhouse gas pollution which has resulted from the industrial revolution and population explosion, have propelled our climate from one of its coldest decades to one of its hottest, in just a single century.
Using data from 73 sites around the world, including fossils of tiny marine organisms, ice cores and tree rings, scientists have been able to reconstruct Earth’s temperature history back to the end of the last Ice Age, providing the most complete and comprehensive reconstruction of global temperatures to date and is generally consistent with the existing data from both shorter term global studies and regional studies. The study has revealed that the planet today is warmer than it has been during 70 to 80 percent of the time over the last 11,300 years.
“We already knew that on a global scale, Earth is warmer today than it was over much of the past 2,000 years. Now we know that it is warmer than most of the past 11,300 years. This is of particular interest because the Holocene spans the entire period of human civilization.”
said lead author, post-doctoral researcher Sean Marcott from Oregon State University's College of Earth, Ocean and Atmospheric Sciences who noted that previous research on past global temperature change has largely focused on the last 1,500 years.
This new research, which extends the reconstruction of global temperatures back to the end of the last Ice Age puts today’s climate into a much broader context.
Climate scientist Michael Mann from Penn State University published a line graph in 1999 depicting global temperatures over the last 1,500 years, which was about as far back as reliable data from tree rings could provide. The graph has been colloquially known amongst climate scientists as the "hockey stick" graph because it shows a massive, unmistakable uptick in temperature beginning with the industrial revolution and clearly linked to human activity, rather than any natural cycle. Throughout the last 14 years it has remained one of the most compelling and graphical representations of our undeniable effect on the global clilmate.
The earth's climate has been warming since the industrial revolution. This has been an uncontested observation for over a century, however, there has been debate as to how that fits into the broader picture of historical, global temperature patterns. Exactly how warm is the climate now compared with the rest of the Holocene period, the geographical epoch which began around 12,000 years ago and in which we are now living? That is the question that a research team from Oregon State University set out to discover.
To answer that question, the researchers have gone back much further into history, constructing a record of global mean surface temperatures for the entire Holocene period, covering more than 11,000 years and using a variety of global land- and marine-based proxy data, other indications which can stand in for a thermometer reading. By expanding the tools they used to gain climate data to include ice cores and the composition and distribution of fossilized ocean shells that have been unearthed by oceanographers, the researchers were able to provide a much broader climate picture for the earth.
Existing research has shown that certain chemical tracers in fossilized shells create a direct link to the temperature at the time they were created. Oxygen isotopes in fossilized plankton, for instance, can allow scientists to calculate that the shell of a particular plankton was formed during a period when Greenland was completely without ice.
The temperature pattern which emerged from this research, shows a rise as the world emerged from the last ice age, warm conditions until the middle of the Holocene, and a cooling trend over the next 5000 years that culminated around 200 years ago in the Little Ice Age. Temperatures have risen steadily since then, leaving us now with a global temperature higher than those during 90% of the entire Holocene period.
According to the NY Times website, scientists say that if natural factors were still governing the climate, the Northern Hemisphere would probably be destined to freeze over again in several thousand years.
"We were on this downward slope, presumably going back toward another ice age,” Dr. Marcott said.
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The earth's climate cooled by around 1.3 degrees Fahrenheit over a period of 5,000 years. It has taken us just 100 years to reverse that 5,000 year trend with global temperatures rising by the same amount, 1.3 degrees Fahrenheit in the last 100 years alone after the low of 200 years ago, at the end of the Little Ice Age. Orbital forcings and temperatures did rise sharply at the end of the ice age, but they then remained relatively stable for about 5,000 years at about 0.6°C above the temperature of the last 1,500 years.
The largest temperature changes are visible in the northern hemisphere, where there are more land masses and greater human populations, lending further support to the theory of anthropogenic global warming.
Of even more concern are projections of global temperature for the year 2100, when virtually every climate model evaluated by the Intergovernmental Panel on Climate Change (IPCC) shows that temperatures will exceed the warmest temperatures during that 11,300-year period known as the Holocene – under all plausible greenhouse gas emission scenarios.
Peter Clark, an OSU paleoclimatologist and co-author on the Science article, said many previous temperature reconstructions were regional in nature and were not placed in a global context. Marcott led the effort to combine data from 73 sites around the world, providing a much broader perspective.
“When you just look at one part of the world, the temperature history can be affected by regional climate processes like El Niño or monsoon variations,” noted Clark. “But when you combine the data from sites all around the world, you can average out those regional anomalies and get a clear sense of the Earth’s global temperature history.”
What that history shows, the researchers say, is that over the past 5,000 years, the Earth on average cooled about 1.3 degrees (Fahrenheit) – until the past 100 years, when it warmed ̴ 1.3 degrees (F). The largest changes were in the northern hemisphere, where there are more land masses and greater human populations.
Climate models project that global temperature will rise another 2.0 to 11.5 degrees (F) by the end of this century, largely dependent on the magnitude of carbon emissions.
“What is most troubling,” Clark said, “is that this warming will be significantly greater than at any time during the past 11,300 years.”
Marcott said that one of the natural factors affecting global temperatures over the past 11,300 years is gradual change in the distribution of solar insolation associated with Earth’s position relative to the sun.
“During the warmest period of the Holocene, the Earth was positioned such that Northern Hemisphere summers warmed more,” Marcott said. “As the Earth’s orientation changed, Northern Hemisphere summers became cooler, and we should now be near the bottom of this long-term cooling trend – but obviously, we are not.”
The modern rise that has recreated the temperatures of 5,000 years ago is occurring at an exceedingly rapid clip on a geological time scale, appearing in graphs in the new paper as a sharp vertical spike, reports the NY Times. If the rise continues apace, early Holocene temperatures are likely to be surpassed within this century, Dr. Marcott said.
Dr. Mann pointed out that the early Holocene temperature increase was almost certainly slow, giving plants and creatures time to adjust. But he said the modern spike would probably threaten the survival of many species, in addition to putting severe stresses on human civilization.
“We and other living things can adapt to slower changes,” Dr. Mann said. “It’s the unprecedented speed with which we’re changing the climate that is so worrisome.”
Clark said that other studies, including those outlined in past IPCC reports, have attributed the warming of the planet over the past 50 years to anthropogenic, or human-caused activities – and not solar variability or other natural causes.
“The last century stands out as the anomaly in this record of global temperature since the end of the last ice age,”
said Candace Major, program director in the National Science Foundation’s Division of Ocean Sciences, which co-funded the research with NSF’s Division of Atmospheric and Geospace Sciences.
“This research shows that we’ve experienced almost the same range of temperature change since the beginning of the industrial revolution as over the previous 11,000 years of Earth history – but this change happened a lot more quickly.”
The research team, which included Jeremy Shakun of Harvard University and Alan Mix of Oregon State, primarily used fossils from ocean sediment cores and terrestrial archives to reconstruct the temperature history. The chemical and physical characteristics of the fossils – including the species as well as their chemical composition and isotopic ratios – provide reliable proxy records for past temperatures by calibrating them to modern temperature records.
Using data from 73 sites around the world allows a global picture of the Earth’s history and provides new context for climate change analysis.
“The Earth’s climate is complex and responds to multiple forcings, including CO2 and solar insolation,” Marcott said. “Both of those changed very slowly over the past 11,000 years. But in the last 100 years, the increase in CO2 through increased emissions from human activities has been significant. It is the only variable that can best explain the rapid increase in global temperatures.”
The study, by researchers at Oregon State University and Harvard University was funded by the National Science Foundation’s Paleoclimate Program. Marcott received his Ph.D. in geology in 2011 from OSU.
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Marcott, S. A. (2013-03-08) A Reconstruction of Regional and Global Temperature for the Past 11,300 Years. , 339(6124), 1198-1201. DOI: 10.1126/science.1228026