Effect of arctic amplification on temperature rises above the paris limits

Faster Arctic warming hastens 2C rise by eight years

MUNICH – Faster warming in the Arctic will be responsible for a global 2-degree Celsius temperature rise being reached eight years earlier than if the region was warming at the average global rate, according to researchers from University College London (UCL).

The Arctic is currently warming nearly four times faster than the global average rate. The new study, published in the EGU open-access journal Earth System Dynamics, aimed to estimate the impact of this faster warming on how quickly the global temperature thresholds of 1.5C and 2C, set down in the Paris Agreement, are likely to be breached.

Effect of arctic amplification on temperature rises above the paris limits
Figure. Effect of Arctic amplification on temperature rises above the Paris limits in CMIP6 models. (a) Observed (dashed lines) and CMIP6 projected global mean temperature anomalies relative to pre-industrial with and without Arctic amplification of warming. The CMIP6 projections are constructed from the first (or only) ensemble member of each model. All temperatures anomalies are 20-year, centred, rolling means. Projections are from the SSP2-4.5 emissions pathway and are scaled to diverge from the observed temperature anomaly, which results in a divergence in 2013, half the window-length prior to the present day. Central lines indicate multi-model means, shaded regions represent the spread between the 10% and 90% intervals. (b) Number of years earlier in which each CMIP6 model breaches the 1.5◦C temperature threshold as a result of amplified warming in the Arctic. For models with multiple ensemble members available, error bars show the range of this value across the full ensemble, and dots show the mean value, and numbers in brackets following the model names indicate the number of ensemble members used. For the multi-model ensemble, the box shows the 25th to 75th percentile range, and whiskers show the 5th to 95th percentile range. © Distributions of crossing years in the multi-model ensemble for the two temperature thresholds, with and without Arctic amplification. The box plots are defined as in (b). The mean number of years early with which each temperature threshold is crossed due to Arctic amplification is labelled. Both labelled differences are significant at a 99% confidence level using a one-sided t-test.

To do this, the research team created alternative climate change projections in which rapid Arctic warming was not occurring. They then compared temperatures in this hypothetical world with those of the “real-world” models and examined the timing with which the critical Paris Agreement thresholds of 1.5C and 2C were breached.

They found that in the models without fast Arctic warming, the thresholds were breached five and eight years later respectively, than their “real-world” projected dates of 2031 and 2051. In addition, they found that disproportionately fast Arctic warming, known as Arctic amplification, added disproportionate uncertainty to forecasts, as the variation in model projections for the region is larger than for the rest of the planet.

Alistair Duffey, UCL Earth Sciences and lead author of the study, said: “Our study highlights the global importance of rapid Arctic warming by quantifying its large impact on when we are likely to breach critical climate thresholds. Arctic warming also adds substantial uncertainty to climate forecasts.”

“These findings underscore the need for more extensive monitoring of temperatures in the region, both in-situ and via satellites, and for a better understanding of the processes occurring there, which can be used to improve forecasts of global temperature rise.”

The study does not attempt to quantify the ways in which Arctic warming affects the rest of the world, for instance through the retreat of sea ice which helps to keep the planet cool, but instead estimates the direct contribution of Arctic warming to global temperature increases.

Co-author Julienne Stroeve (UCL Earth Sciences, the University of Manitoba, Canada, and the U.S. National Snow and Ice Data Center) explained: “While our study focuses on how Arctic warming affects global temperature change, the local impacts should not be overlooked. A 2C temperature rise globally would result in a 4C annual mean rise in the Arctic, and a 7C rise in winter, with profound consequences for local people and ecosystems.

“In addition, rapid warming in the Arctic has global consequences that we do not account for in this study, including sea level rise and the thawing of permafrost which leads to more carbon being released into the air.”

Co-author Robbie Mallett (University of Manitoba and Honorary Research Fellow at UCL Earth Sciences) said: “Arctic climate change is often overlooked by politicians because most of the region is outside national boundaries. Our study shows how much the Arctic impacts global targets like the Paris Agreement, and hopefully draws attention to the crisis that’s already unfolding in the region.”

Arctic amplification, which is strongest in the winter months, is caused by several factors. One is the retreat of sea ice, meaning more sunlight (and heat) is absorbed by water instead of being reflected back into space. Another factor is less vertical mixing of air in the poles than in the tropics, which keeps warmer air close to the Earth’s surface.

For the study, researchers looked at an ensemble of 40 climate models that informed the UN’s 2021 climate change report*. These models divide Earth’s surface into a three-dimensional grid of cells, modelling physical processes occurring within each cell. The research team modified the output of the models to create an alternative world in which rapid Arctic warming was not occurring, by setting the rate of change of temperature in the region North of 66° North equal to that of the rest of the planet. They looked at how the removal of rapid Arctic warming would affect temperature projections in a plausible intermediate emissions scenario and calculated the average temperature projection across all models.

In addition, they looked at how removing rapid Arctic warming from the models would affect more pessimistic or optimistic scenarios. For example, in a more optimistic scenario, where emissions are cut sharply and net zero is reached shortly after 2050, Arctic amplification causes a 7-year difference in the time of passing 1.5°C. Temperature projections for the Arctic varied more substantially between the models than for other parts of the globe, accounting for 15% of the uncertainty in projections, despite the region only making up 4% of the global surface area.

The 1.5C and 2C limits are regarded as having been breached when average global temperatures over a 20-year period are 1.5C or 2C higher than in pre-industrial times. The goal of the Paris Agreement, an international treaty, is to keep the global average temperature to “well below 2°C above pre-industrial levels” and pursue efforts “to limit the temperature increase to 1.5°C”. The Arctic is thought to have warmed by 2.7C since the pre-industrial era, and this warming is believed to have accelerated since the start of the 21st century.

The study was supported by the Natural Environment Research Council (NERC), the European Space Agency (ESA), and the Canada 150 Research Chairs Program.