Climate: How Warm Is It?

According to three recent papers, it’s a field day for the heat.

Sure, global temperatures have been on the rise for the past 100 years or so. But, some people ask, is that unusual? Are temperatures today significantly different from those of the recent and distant past?

These are reasonable questions, but questions that have been basically answered (yes and yes) by Michael Mann and company’s hockey stick reconstruction and the largely confirmatory analyses that followed. But temperature reconstructions like Mann’s are difficult, and we all know how controversial that whole hockey-stick thing turned out to be, so new, independent evidence is always welcome.

In that vein, it is interesting to note the recent publication of three papers, each taking a completely different approach to the subject.

A Record Going Back 11,300 Years

The invention of the mercury thermometer is credited to Gabriel Fahrenheit in the early part of the 18th century. Reliable records of atmospheric temperatures using thermometers only date back to the mid- to late-1800s. And so if you want to know something about global atmospheric temperatures before then, you’re going to have to use something besides temperature measurements. (The temperature record obtained from thermometers is known as the instrumental record.) Scientists have developed a wide variety of proxies to infer the temperatures before the instrumental record. These include ice cores, tree rings, pollen, ocean sediments and corals, to name a few.

Up until recently, the temperature reconstructions have been limited to the past 1,500 years or so. (Michael Mann’s initial reconstruction, which produced the hockey stick, went back about 600 years.) In March, Shaun Marcott of Oregon State University and co-authors reported on a new temperature analysis using 73 separate proxy records that go back 11,300 years, near the beginning of the current interglacial known as the Holocene or recent period.

Their analysis indicates that the global mean temperature for 2000–2009 is higher than at any time over the past 5,000 years but does not exceed the peak temperatures during the early half of the Holocene. Current temperatures from the last decade, they report, exceed at least 72 percent of the temperatures they inferred for the Holocene. And, the authors conclude:

“Climate models project that temperatures are likely to exceed the full distribution of Holocene warmth by 2100. … regardless of the greenhouse gas scenario considered. … By 2100, global temperatures will probably be 5 to 12 standard deviations above the Holocene temperature mean.”

Tropical Ice Tells a Similar Story: Today’s Temps Are Hot

When it comes to the world’s high-altitude tropical glaciers, Lonnie Thompson of Ohio State University is the man. He’s been documenting the retreat of those glaciers for more than three decades. His most recent paper published online last week in the journal Science with co-authors documents a startling result.

Peru map

(Source: OSU)

By carbon-dating the remains of dead plants that had been covered by ice, Thompson and his co-authors were able to determine the age of the ice at the edge of Peru’s melting Quelccaya ice cap. Using this method, the authors estimate that the ice currently melting at the edges of the retreating margin is about 6,300 years old. From this finding we can extrapolate that temperatures at this location in the Andes are now so warm that they are starting to melt ice that has heretofore continuously persisted for more than 6,000 years.

Interestingly, this result is qualitatively consistent with the Marcott et al conclusion that current temperatures are warmer than at any time over the past 5,000 years or so.

A 500,000-Year Record In Siberia

Also writing in the journal Science, Anton Vaks of the University of Oxford and co-authors report on a long-term analysis of climatic conditions over Siberia using cave formations known as speleotherms. Speleotherms form from precipitates in water and thus only when temperatures are above the freezing point of water. As you might imagine, speleotherm formation is a relatively rare occurrence in Siberia (as it is currently configured). And evidence of its formation in Siberian caves can therefore be used  as a proxy for warm periods in which temperatures have risen above the freezing point of water.

By sampling and dating the layers of speleotherms in six caves spanning a north-south transect from Siberia to the Gobi Desert, Vaks et al were able to identify the warm periods in Siberia over the past 500,000 years. And you know what? Those warm periods determined by the speleotherm formation generally match warm periods from other climatic reconstructions.

Why is this of interest? In its northern latitudes Siberia is home to a huge fraction of the 1,700 gigatons of carbon stored as permafrost. The frozen landscapes isolate the carbon from the atmosphere thus preventing it from decaying, much like placing a pound of chopped beef in your freezer keeps it from going bad. But if speleotherm formation requires liquid water, it suggests that periods of speleotherm formation were periods when the permafrost had melted to some extent, making the stored carbon available to be metabolized and emitted into the atmosphere as two global warming gases, carbon dioxide and methane.

But perhaps the most interesting result came from the data gathered from the northernmost (and therefore coldest) cave in their study — Lenskaya Ledyanaya — just north of 60 degree North latitude. The authors found that the last time this particular cave had any speleotherm formation was about 450,000 years ago, when the Earth was in one of its warm phases of the cycle of cold (glacial) and warm (interglacial) periods that dominated our millennial-scale climate for the past two million years. Of course, this cave is not experiencing speleotherm formation now either. So the good news is that current temperatures have not exceeded those experienced 450,000 years ago.

But here’s the kicker: global temperatures during the warm period 450,000 years are estimated to have been only about 1.5 degrees Celsius (2.7 degrees Fahrenheit) warmer than preindustrial temps. Two takeaways:

  1. 1.5 degrees Celsius is less than the 2 degree threshold that the world community has identified to avoid “dangerous climate change.” Perhaps 2 degrees is not protective enough?
  2. Temperatures today are about 0.8 degrees Celsius (1.5 degrees Fahrenheit) above preindustrial temperatures. In other words, according to Vaks et al, we are about halfway to potentially melting a substantial amount of permafrost and releasing the carbon stored there.

The bottom line: we may not be exactly sure what’s going on, but it’s pretty clear that human activities are driving a pretty unusual — one might say extreme — temperature spike.

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