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07-10-2024

Polar regions are rapidly warming with global consequences

Polar regions are warming at a much faster rate compared to lower latitudes, a phenomenon referred to as “polar amplification.” 

The Intergovernmental Panel on Climate Change (IPCC) has reported a ~5 °C rise in Arctic temperatures over the 20th century, with the highest rates of ~1 °C per decade since the 1980s. 

This rapid warming not only impacts the ecosystems of these regions but also has significant global consequences.

Vicious cycle of ice melt

Ice and sea ice in both the Arctic and Antarctic play a crucial role in regulating climate through ice-albedo feedback. Ice reflects incoming solar radiation, helping to maintain cooler temperatures and preserve ice masses. 

As global temperatures rise, ice melts, exposing darker land and sea surfaces that absorb more solar radiation. This absorption warms the environment further, leading to more ice melt and perpetuating the cycle.

Atmospheric moisture and cloud cover

While the ice-albedo feedback is a significant concern, it is not the only mechanism that could lead to an ice-free world. Changes in atmospheric moisture and cloud cover can also affect radiation levels. 

The exact contributions of albedo and atmospheric processes to polar amplification remain uncertain, making it difficult to predict the extent of future polar warming and ice melt accurately.

Driving force of warming polar regions

To gain more insight, scientists examine Earth’s geological past, particularly the Eocene epoch (~48–56 million years ago), a period when the planet was ice-free. This period helps isolate atmospheric processes as the driving force behind polar amplification, excluding large ice-albedo changes. 

Research published in Climate of the Past has now quantified polar amplification during the Eocene, particularly during short-lived warming events known as hyperthermals, such as the Paleocene-Eocene Thermal Maximum (~56 Ma) and the Eocene Thermal Maximum 2 (~54 Ma).

“At present, polar amplification is a key uncertainty in predictions of future climate warming, with latest IPCC model estimates ranging from factor two to four. 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,” said Chris Fokkema, a doctoral researcher at Utrecht University.

Past climate of polar regions

The researchers used cell membrane lipid biomarkers from Nitrososphaera microorganisms to reconstruct sea surface temperatures during the Eocene. 

This method, known as the TEX86 paleothermometer, involves analyzing the membrane lipids of archaea, which produce more rings in their lipid molecules at higher temperatures to maintain membrane rigidity. 

The lipid molecules are well-preserved in sediments, allowing scientists to extract and analyze them from ancient marine sediments.

“This method is based on analyzing the membrane lipids of archaea 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,” Fokkema explained.

The role of orbital forcing 

The team analyzed sediments from Ocean Drilling Program cores in the tropical Atlantic Ocean off the coast of North Africa. 

They found that temperature variability in the tropics was time-equivalent with that at high latitudes during hyperthermals and across Milankovitch cycles, suggesting global temperature changes driven by variations in Earth’s orbit. 

The scientists attribute these changes to orbital forcing, which affects the carbon cycle by influencing carbon dioxide levels in the atmosphere.

Climate models underestimate polar warming 

High latitude ocean temperatures during hyperthermals varied about twice as much as tropical surface oceans, indicating strong atmospheric feedbacks. 

This finding suggests that current climate models may underestimate polar amplification, implying that future warming in the Arctic and Antarctic could be greater than predicted.

“Our new study shows that the current warming is already on the same scale as some of these hyperthermals, which strongly impacted climate and oceans. 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,” said Fokkema.

“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.”

Understanding polar amplification is crucial for predicting how permafrost thawing and ice sheet melting will impact sea level rise and the global carbon cycle. 

The insights gained from studying the Eocene period provide valuable information for refining climate models and improving our predictions of future climate change.

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