Polar warming may be underestimated by climate models, ~50 million year old climate variability suggests

Polar regions are known to be warming at an enhanced rate compared to lower latitudes, with the Intergovernmental Panel on Climate Change citing a ~5 °C increase in air temperature over Arctic land masses during the 20th century and the highest rates of ~1 °C per decade since the 1980s. Clearly, this so-called "polar amplification" of warming, defined as the ratio of high-latitude (>60 ºN/S) to low-latitude (<30 ºN/S) warming, is a significant issue not only affecting the organisms calling polar regions home, but also how this impacts the rest of the planet.

Specifically, land and sea ice in the Arctic and Antarctic has a prominent role in modulating climate through ice-albedo feedbacks. This occurs due to ice being 'white,' thus reflecting incoming solar radiation and helping to maintain both temperature and ice masses.

However, as climate warms and ice melts, more of the comparatively darker land and sea surface is exposed, which absorbs the incoming solar radiation, warming the ambient environment further and causing additional melting, and so the loop continues.

Such a feedback loop poses concerns for the future of our polar regions, but it is not the only mechanism by which an ice-free world can occur, as changes in atmospheric moisture and clouds can impact radiation. It is, in fact, unclear how much albedo and atmospheric processes contribute to polar amplification, which also poses large uncertainties for the magnitude of future polar warming and ice melt.

Fortunately, we can look to the geological past to analyze a period in which Earth was ice free. This excludes large ice-albedo changes as a driving force, so that all polar amplification was caused by atmospheric processes. Such a period is known from the Eocene, particularly the earliest part of the Ypresian, which occurred ~48–56 million years ago (Ma), and saw mean surface temperatures rising 10–16 °C above pre-Industrial Revolution levels.

New research published in Climate of the Past has now quantified polar amplification of global climate variability that was caused by variations in the Earth's orbit around the sun (Milankovitch cycles), particularly during a succession of short-lived (
Doctoral researcher Chris Fokkema, of Utrecht University, Netherlands, explains the importance of studying polar amplification, particularly during this interval of Earth's history, "At present, polar amplification is a key uncertainty in predictions of future climate warming, with latest IPCC model estimates ranging from factor 2 to 4. Polar warming has global consequences, because ice sheet melting causes sea level rise, but also because permafrost thaw can release large quantities of carbon dioxide.

"Two factors dominantly control polar amplification—ice-albedo and atmospheric feedbacks—which cannot be easily separated since both are active in the modern era. Data to validate theory will have to come from paleoclimate. To quantify the non-ice-albedo-related polar amplification feedbacks, here we reconstructed polar amplification in the ice-free climate state of the early Eocene.

"Previously, this was only possible on multi-million year timescales (very far from present climate change) because no tropical sea surface temperature record of higher resolution was available. Here, we solved this issue by generating a ~4,000-year resolution sea surface temperature record at a tropical location."

For these hyperthermal intervals, the temperature variations of the high latitude oceans were already known from temperature reconstructions of the deep ocean, which, as in the modern era, derived from high latitudes during the Eocene.

The missing piece to assess polar amplification was Eocene sea surface temperature reconstructions from the tropics. Therefore, Fokkema and colleagues have used cell membrane lipid biomarkers from Nitrososphaera microorganisms to resolve this.

This relationship of microorganisms as recorders of temperature from the geological past is based upon the TEX₈₆ paleothermometer.

Fokkema further reveals how this process works: "This method is based on analyzing the membrane lipids of archaea (Nitrososphaera) that live near the ocean surface. The simple principle behind this method is that these archaea produce relatively more rings in their membrane lipid molecules at higher ambient temperatures, to retain their membrane rigidity. These molecules are preserved very well in the sediments after they reach the ocean floor, which makes it possible for us to extract them out of ancient marine sediments.

"We measured the relative occurrence of these different structures of membrane lipids in our sediment samples, which was then related to a sea surface temperature by using present-day relationships. We chose the TEX₈₆ method because these lipids were abundant in these unique sediments and the method works well to reconstruct the warm temperatures >36 ºC of the early Eocene tropics."

The scientists analyzed sediments from Ocean Drilling Program cores in the tropical Atlantic Ocean off the coast of North Africa (Site 959) and found that temperature variability in the tropics was time-equivalent with that at high latitudes during the hyperthermals, but also across Milankovitch cycles, providing strong evidence that these variations were global.

This proves a link between the orbital cycles, global temperature variability and the carbon content of the atmosphere. Given all of this, the scientists attribute orbital forcing as a major driver of changes in the carbon cycle, which relates to the input of carbon dioxide into the atmosphere that exacerbates global warming. They cite soils, peats, permafrost and methane hydrates as potential major sources of the carbon release.

The research team also identified that high latitude ocean temperatures during hyperthermals and orbital cycles varied about two times (1.7 to 2.3) stronger than the tropical surface ocean, therefore implying warmer polar regions. So even during the ice-free Eocene, high latitudes warmed and cooled twice as much as tropical regions, without ice-albedo feedbacks, suggesting strong atmospheric feedbacks.

Furthermore, because of polar amplification, the temperature gradient between the pole-derived deep ocean waters and tropical sea surface temperature was lower during hyperthermals. Global mean sea surface temperature for the Eocene hyperthermals increased ~1–1.5 °C during the events, which is comparable to modern rates of ~1 °C.

Fokkema and colleagues then compared this Eocene polar amplification factor with that calculated by the same climate models that are used to project future climate change, but then adapted for Eocene geography and ice-free conditions.

This showed that models underestimate polar amplification (1.1–1.3x). Consequently, this may imply that models underestimate the impact of warming in the Arctic and Antarctic. Understanding how polar amplification may progress in the future is vital to assess how thawing of permafrost and melting of ice sheets may influence sea level rise, as well as the carbon cycle.

"Our new study shows that the current warming is already on the same scale of some of these hyperthermals, which strongly impacted climate and oceans," Fokkema concludes. "By comparing this new record to previously published open ocean bottom water temperatures we were able to calculate polar amplification on short timescales during multiple orbitally-forced global temperature changes.

"We found a polar amplification factor of ±2. Interestingly, this is slightly larger than the polar amplification that climate models predict in an ice-free, early Eocene world, which may imply that climate models underestimate polar warming for the future."

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