Session B65: Extreme Events, Tipping Points, and Abrupt Changes in the Climate System

Heat released from the depths of the Arctic Ocean: amplified warming and tipping points

Surface temperature observations indicate considerably faster warming in the Arctic than in the rest of the globe. This raises questions regarding why the warming has been so concentrated in the Arctic and how the rapidly changing Arctic climate will evolve in the future. The Arctic Ocean, which covers much of the Arctic region, is characterized by an ice-covered layer of cold and relatively fresh water above warmer and saltier waters below. It is estimated that enough heat is stored at depth in the Arctic Ocean to melt all the Arctic sea ice many times over. But this heat has historically remained trapped at depth: the seawater density differences are dominated by salinity, making the vertical stratification stable, and the sea ice cover damps wind-generated internal waves that could otherwise mix the warm waters up to the surface. In this talk, I will discuss the implications of this potentially precarious state of the Arctic Ocean as the climate warms. The talk will draw on idealized physical models with varying levels of complexity, as well as more comprehensive global climate models and findings from in situ observations. In the first part of the talk, I will discuss a proposed physical mechanism by which changing vertical heat fluxes in the Arctic Ocean contribute to the observed Arctic amplification of global warming. In the second half of the talk, I will present a novel positive feedback process involving the release of subsurface heat in the Arctic Ocean. Idealized model results show that this feedback process can give rise to a hysteresis window bounded by saddle-node bifurcations, featuring an abrupt "tipping point" under global warming when the bifurcation point is crossed. The hysteresis occurs for only a limited range of plausibly realistic parameters, however, and questions remain regarding the likelihood that this potential tipping point could occur under global warming during the coming century. Nonetheless, even in the absence of a tipping point, this positive feedback could substantially accelerate the melt of Arctic sea ice.   

Polar Transitions: The Arctic Ocean's Diffusive Staircase

The Arctic Ocean, and its reflective sea ice, is of key importance for global climate change. In the Arctic Ocean, warmer waters can underlie colder waters - thus, a reservoir of heat sits at depth. These warm, salty waters are sourced from the Atlantic and subduct below cooler, fresher waters in the Arctic Basin. One way this stored ocean heat is transported vertically is via diffusive convection, a process that may occur if ocean temperature and salinity gradients are both positive with depth. This diffusive-convective mixing mechanism is evidenced by distinctive ``staircase'' features, where approximately meter-thick layers are separated by cm-thick interfaces in temperature and salinity. These staircase features are found across the Arctic Ocean, most notably in the Beaufort Gyre, a region of the western Arctic. However, how these staircases form, and what ultimately governs staircase layer thicknesses is not well known. In this talk, I will describe how the thickness of these layers shows a sharp spatial transition across the Beaufort Gyre, using an observational data set. I will then relate this transition to the influx of a warm Atlantic water pulse as well as the background oceanographic circulation. This work describes how staircases propagate across the Arctic, and how staircase features may give insight into Arctic climate change.

    

Assessing the long-term dynamics and stability of Arctic ice shelves with periodic roll structure

The breakup and drift of multiyear landfast sea ice from the Arctic triggered several episodes of abrupt climate change (including deglacial cooling periods, Condron et al., 2020) after the end of the Last Glacial Period (LGP). Though most of this sea ice disintegrated during deglaciation, the few ice shelves that remain in the Arctic exhibit one common feature: large rolls on the surface of the ice. Using physical models, we have shown that the vibrational modes of such ice shelves contain band gaps in the frequency domain, implying that these ice shelves prevent ambient ocean sea swell waves of certain frequencies from flexing and breaking the ice. Based on these findings, we create a stochastic model of ice shelf decay due to random calving events whose probability is modulated by the presence of surface rolls. We examine fluctuations in ice extent on the millennial scale timespan since the LGP, also accounting for the growth of the ice shelf by snow accumulation and lateral addition of marine ice. In the presence of the same external factors, ice shelves with surface rolls are more likely to achieve a stable mass-balance while ice shelves without these features are eventually driven into extinction. This model demonstrates that certain ice shelves have a historical “fitness” for survival which is important for understanding the interaction between ice and climate.

Condron, A., Joyce, A. J., Bradley, R. S. (2020). Arctic sea ice export as a driver of deglacial climate. Geology, 48 (4): 395–399.    

Bifurcations in Ice Sheet Behavior and the Implications for Projections of Future Sea Level Rise

Sea level has been rising globally since at least the early 20^th century. This rise can be explained almost entirely by the expansion of warming seawater and melting of land-based glaciers and ice sheets. More recently, the contribution of melting from the Greenland and Antarctic ice sheets to global sea level rise has increased dramatically. This increase in the rate of ice sheet melt, and corresponding acceleration in the rate of global sea level rise has largely been explained by two factors: (1) increased surface melt on the Greenland Ice Sheet, and (2) increased ice flow speeds and discharge to the ocean (through melting and ice fracture) from both ice sheets. In this talk, I will review explanations for these dramatic changes, using canonical bifurcation theories in ice sheet dynamics. I will then review more recent work adapting ideas from piecewise bifurcation theory and statistical physics to further understand enigmatic aspects of past ice sheet changes and projections of future changes. I will conclude by explaining how these ideas from physics are being used to developing better numerical models of ice sheet evolution, and improve the accuracy of future sea level projections. Such projections are already used by coastal communities as critical tools in planning for future sea level rise in order to avoid trillions of dollars in potential losses from coastal flooding globally.   

Non-Gaussian stochastic dynamical model for the El Niño southern oscillation

A nonautonomous stochastic dynamical model approach is developed to describe the seasonal to interannual variability of the El Niño southern oscillation (ENSO). We determine the model coefficients by systematic statistical estimations using partial observations involving only sea surface temperature data. Our approach reproduces the observed seasonal phase locking and its uncertainty, as well as the highly non-Gaussian statistics of ENSO. Finally, we recover the intermittent time series of the hidden processes, including the thermocline depth and the wind bursts.   

Feedback-enabled transient growth can trigger climate tipping points

We study the mitigation of climate tipping point transitions using an energy balance model. The evolution of the global mean surface temperature is coupled with the CO2 concentration through the green house effect. We model the CO2 concentration with a stochastic delay differential equation (SDDE), accounting for various carbon emission and capture scenarios. The resulting coupled system of SDDEs exhibits a transient growth of CO2 concentration which may result in a drastic surface temperature increase of six degrees Celsius, even if the concentration decays asymptotically. In addition, we derive a rigorous upper bound for the CO2 evolution which quantifies its transient and asymptotic growths, and provides sufficient conditions for evading this climate tipping point. Combining this upper bound with Monte Carlo simulations of the stochastic climate model, we investigate the emission reduction and carbon capture scenarios that would avert the tipping point.   

A New Theory for Heat Extremes in a Changing Climate

Heatwaves damage societies worldwide and are intensifying with global warming. An accurate projection for future heat extremes requires a fundamental understanding of the physical mechanisms. Here, we demonstrate the role of convective instability in limiting extreme temperatures and wet-bulb temperatures (heat stress) over land. We then provide a theory for an upper bound of midlatitude surface temperatures and a theory for tropical extreme wet-bulb temperatures. Finally, we discuss how heat extremes will change with global warming based on the theory.   

Atmospheric circulation compounds anthropogenic warming and extreme climate impacts in Europe

Diagnosing dynamical changes in the climate system, such as those in atmospheric circulation patterns, remains challenging. Here, we study 1950-2021 trends in the frequency of occurrence of atmospheric circulation patterns over the North Atlantic. Roughly 7% of atmospheric circulation patterns display significant occurrence trends, yet they have major impacts on surface climate. Increasingly frequent patterns drive heatwaves across Europe, and enhanced wintertime storminess in the northern part of the continent. Over 91% of recent heatwave-related deaths and 33% of high-impact windstorms in Europe were concurrent with increasingly frequent atmospheric circulation patterns. Atmospheric patterns which are becoming rarer correspond instead to wet, cool summer conditions over northern Europe and wet winter conditions over continental Europe. The combined effect of these circulation changes is that of a strong, dynamically-driven year-round warming over most of the continent and large regional and seasonal changes in precipitation and surface wind.   

Inseparable Link between Air pollution and Climate Change

Air Pollution- the leading environmental risk factor for mortality, 7 million premature deaths and as many as 23 million emergency room visits in 2015. It is a silent killer and expected to get worse. It is given very little attention by policy makers because the death due to Air pollution is not as dramatic and media attention grabbing as fthose directly linked to climate.

 

Often called short-lived climate pollutants (SLCPs), particulate matter tropospheric ozone and methane contribute to both the warming of the climate as well as air pollution.  Air pollution can significantly affect the water cycle. By reducing solar radiation reaching the earth particulate matter can affects the rate at which water evaporates and moves into the atmosphere. and affect clouds' formation.

 

Climate change induced hotter summers come with an increase in stagnation events stationary domes of hot air that can cause air pollutants to get trapped and persist in the lower atmosphere.  Climate warming, causes the Earth to experiences more extreme weather, such as heat waves and drought, which can negatively impact air quality as an increase in ground ozone levels. Although they may seem to be two very different issues by reducing air pollution we also protect the climate.

 

While air pollution is a universal problem and impacts everyone, the main victims are low-income communities, communities of color and people in low- and middle-income countries. Our work is focused on laboratory measurement of optical and chemical properties of biomass burning emissions from biomass fuels from Africa to understand their impact on climate and health. The populations in African megacities are growing at the fastest rates of all global regions. Africa is projected to have the fastest urban growth rate in the world: by 2050, Africa's cities will be home to an additional 950 million people, and the continent's population is anticipated to be primarily urban by 2035. The acute levels of indoor and outdoor air pollution in Africa have already become the most significant environmental contributor to premature death, outpacing both malaria and HIV with 1.1 million premature deaths in 2019.

 

The link between air quality and climate change will be discussed and some results of laboratory studies will be presented.