Antarctic Peninsula Warming, Time After Time
Over the past 50 years or so, the Antarctic Peninsula, the northernmost part of the mainland of Antarctica, has experienced rapid warming and the collapse of a number of ice shelves. A new temperature record derived from an ice core drilled on James Ross Island, has triggered a reassessment of what triggered the recent warming trends. This new core provides the best record of climate events on the peninsula going back at least 20,000 years, and may extend back as far as 50,000 years. From this new data a team of researchers has constructed the most detailed history of climate on the Antarctic Peninsula known to science and it has revealed a number of interesting things. Most important of these is the fact that this area undergoes bouts of rapid warming periodically and that things were at least as warm on the peninsula 2,000 years ago. So much for “unprecedented” warming on the Antarctic Peninsula.
That the Antarctic Peninsula has been subjected to warming over the past half century, during which a number of large ice shelves have collapsed, is fairly well documented. Beneath the ice sheet covering the Antarctic Peninsula, there lies a string of bedrock islands that are separated by deep channels. The bottoms of those channels are at depths considerably below sea level and they are joined together by a grounded ice sheet. Tierra del Fuego, the southern most tip of South America, lies only about 1000 km away across the Drake Passage. Note that this is a tongue of land that extends well from the main body of Antarctica and far into the surrounding ocean waters. The main part of the Antarctic continent is not melting and, indeed, has been gaining ice mass in recent decades.
A research team led by Robert Mulvaney, from the British Antarctic Survey, has reported new findings regarding the periodic warming of the Antarctic Peninsula in a letter to Nature, “Recent Antarctic Peninsula warming relative to Holocene climate and ice-shelf history.” This paper presents an extended deuterium-based record of Holocene temperature variations at James Ross Island (JRI), off the northeastern tip of the Antarctic Peninsula. As is often the case, the article’s abstract provides a good overview and summary of the important findings:
Rapid warming over the past 50 years on the Antarctic Peninsula is associated with the collapse of a number of ice shelves and accelerating glacier mass loss. In contrast, warming has been comparatively modest over West Antarctica and significant changes have not been observed over most of East Antarctica, suggesting that the ice-core palaeoclimate records available from these areas may not be representative of the climate history of the Antarctic Peninsula. Here we show that the Antarctic Peninsula experienced an early-Holocene warm period followed by stable temperatures, from about 9,200 to 2,500 years ago, that were similar to modern-day levels. Our temperature estimates are based on an ice-core record of deuterium variations from James Ross Island, off the northeastern tip of the Antarctic Peninsula. We find that the late-Holocene development of ice shelves near James Ross Island was coincident with pronounced cooling from 2,500 to 600 years ago. This cooling was part of a millennial-scale climate excursion with opposing anomalies on the eastern and western sides of the Antarctic Peninsula. Although warming of the northeastern Antarctic Peninsula began around 600 years ago, the high rate of warming over the past century is unusual (but not unprecedented) in the context of natural climate variability over the past two millennia.
In other words, following peak warmth in the early Holocene, roughly corresponding to the Holocene Thermal Maximum, temperatures were stable until about 2,500 years ago. Then a sharp cooling took place. Warming began again about 600 years ago, building to rapid but not unprecedented rates in the past century. This climate record firmly places recent warming in the context of long-term natural variability, and suggests that future warming could destabilize ice shelves southwards along the peninsula. Some sense of the area under discussion can be drawn from the map below.
The 363.9 m long JRI ice core provides a temperature reconstruction that spans the entire Holocene and extends into the last glacial interval. Evidence of the glacial period ice is found in the final 5 m of the JRI ice core. Initial estimates suggested that the record might extend to ~50,000 years ago, although the far end of the ice core has proven muddled and nearly impossible to accurately decode. Even so, the contribution of this new ice core has been invaluable in understanding climate change on the peninsula starting from the earliest part of the Holocene warming. Quoting from the letter:
The Holocene temperature history from the JRI ice core is characterized by an early-Holocene climatic optimum that was 1.3 ± 0.3 °C warmer than present. The magnitude and progression of this early-Holocene optimum is similar to that observed in ice-core records from the main Antarctic continent. A marine sediment record from off the shore of the western Antarctic Peninsula also shows an early-Holocene optimum during which surface ocean temperatures were determined to be ~3.5 °C higher than present. Other evidence suggests that the George VI ice shelf on the southwestern Antarctic Peninsula was absent during this early-Holocene warm interval but reformed in the mid Holocene.
Here is confirmation of the HTO from the southern hemisphere, with local temperatures 1.3 ± 0.3 °C warmer than current conditions. It also indicates that ice sheets may have disappeared altogether during this warm period 9,200 to 2,500 years ago, and only reformed recently (geologically speaking). “The late-Holocene development of ice shelves fed from the northeastern Antarctic Peninsula seems to be related to millennial-scale climate variability in the region,” the authors conclude.
Even more interesting is what happened over the past couple of millennia, a time when the ice core readings are most accurate. After 2,500 years before the present (yr bp), the JRI isotope record documents pronounced cooling to temperatures that were on average 0.7 ± 0.3 °C cooler than the present. Between 800 and 400 yr bp (1150–1550 AD) temperatures may have at times been more than 1.8 ± 0.3 °C cooler than at present. The prominent millennial-scale cooling at JRI is matched by a similarly prominent but warm excursion in marine temperatures to the west of the Antarctic Peninsula. “Thus, although glacial-scale climate changes have been consistent across the whole of the Antarctic Peninsula region, millennial-scale climate variability was particularly strong during the late Holocene and seems to have been characterized by opposing east–west temperature anomalies across the Antarctic Peninsula,” state Mulvaney et al. A 2,000 year climate history is presented in the figure below.
One interesting contrast to note: while SST to the west of the Antarctic Peninsula shows similarities to Northern Hemisphere climate over the past 2,000 yr, the JRI record shows an opposing temperature excursion which demonstrates that the Antarctic Peninsula did not experience a widespread Medieval Warm Period/Little Ice Age sequence comparable to Northern Hemisphere climate at that time. This could be indicative of a north-south polar temperature seesaw much like the one detected during the previous glacial period. The authors note that “the development of this Antarctic-dipole-like feature during the late Holocene coincides with the well-documented maximum in El Niño activity,” so their may be a connection to northern hemisphere climate change as well.
Now we come to the most intriguing discovery—it turns out that the rapid warming of the past 50 years is not “unprecedented” as many climate alarmists would have us believe. The rate of natural warming (and cooling, for that matter) can vary considerably without need for human greenhouse gas emissions. The problem seems to be that climate scientists have previously only looked at human scale cycles or much longer cycles during the previous glacial period. Here is what Mulvaney and colleagues discovered:
The overall rate of pre-anthropogenic temperature increase at JRI from ad 1400 to ad 1850 equates to 0.22 ± 0.06 °C per century. However, there are times in this interval when warming occurred much faster. Using annual-resolution data, trends were calculated for the JRI temperature record since 2,000 yr bp over moving 100-yr intervals stepped in 1-year increments (yielding 1,958 100-year analysis windows). This analysis indicates that rapid warming trends exceeding 1.5 °C per century occurred at JRI during the intervals spanning ad 1518–1621 and ad 1671–1777, and that trends exceeding 1.25 °C per century occurred during the interval ad 296–415.
In short, today's “unprecedented” warming has occurred three times over the past 2,000 years, and between ~9,200 to 2,500 years ago the Antarctic Peninsula may have been consistently warmer than today. So much like the discovery that Greenland's glaciers vary on an 80-100 year cycle, this portion of Antarctica also undergoes periodic warming and cooling spells. These cycles, with temperatures equaling and often exceeding modern conditions, have been going on long before humanity began burning every hydrocarbon in sight.
All the unprecedented melting—be it in the Arctic, Greenland, the Himalaya or the Antarctic Peninsula—has happened before, usually several times since the start of the Holocene. It is worth repeating: Current temperatures are not abnormal, temperatures have been higher than today in places all over the world; the rate of temperature change has been higher before; and the ice that is melting has melted (and re-frozen) before, time after time. The evidence against anthropogenic global warming constituting a crisis continues to mount, yet climate scientists insist that the world is going to hell because of human emissions. Pardon me if I remain skeptical of anthropogenic global warming, for science has not made a compelling case for this bogus theory.
Be safe, enjoy the interglacial and stay skeptical.