Earth System Sensitivity

Annual global temperatures from 1850-2017. The colour scale represents the change in global temperatures covering 1.35°C. Credit: Climate Lab Book, 2018

The Earth System

Earth is a complex dynamic system. Earth system dynamics can be understood in terms of trajectories between alternate states separated by thresholds that are controlled by nonlinear processes, interactions, and feedbacks. For example, over the past 1.2 million years Earth has remained in a state of glacial and interglacial cycles. The current temperature change at 1,2°C above a preindustrial baseline has already pushed Earth out of the next glaciation cycle.

Furthermore, Earth is a water planet and incredibly inert. The time lag between cause and effect, between the heating and the final change in temperature, is large. The full warming effect of a large emission pulse may not be felt for several decades or centuries. As a result, the currently observed change in temperature represents only a part of the eventual expected increase in temperature resulting from already released greenhouse gas emissions.

Exactly where a potential planetary threshold, between a livable state and a hothouse state, might be is uncertain. Steffen et al. (2018) suggests 2°C as the critical limit, stating that passing two degrees could trigger tipping elements in the Earth System that could cascade, triggering further tipping elements, causing rapid warming beyond human control. 

Thus, actions taken over the next decade could significantly influence the trajectory of the Earth System for tens to hundreds of thousands of years and potentially lead to conditions that would be inhospitable to humans and to many other species.

Main point: Earth is tracking a hothouse pathway

Earth System Sensitivity

How the climate system will respond to increasing CO2 levels depends on time-scale and which feedbacks we consider. Taking into account fast feedbacks such as clouds, water vapour, snow cover change, and aerosols we get a climate sensitivity of about 2-4.5°C to a doubling of CO2. But this does not include slow longer-term feedbacks such as ice sheet disintegration, changes in carbon cycle (e.g. permafrost thaw), vegetation cover changes, or changes in oceans ability to store carbon. If we include all feedbacks, both fast and slow, we get a Earth System Sensitivity of 3-6°C.  

Estimated temperature changes from fast and slow feedbacks. Source: Schmidt, 2016

Studies of past climates in Earth's history show that long-term feedbacks play an important role in Earth's overall climate. For example, during the mid-Pliocene some 3-4 million years ago, when global mean temperatures were about 3-4°C warmer than preindustrial and sea levels 10-25 meter higher than today, CO2 levels peaked at 450 ppm. Our current concentration levels stand at 410 ppm CO2, but temperatures have only risen about + 1,2°C, so Earth is likely to warm up at least to similar levels eventually. And we would over millenia have sea-level rise of up to at least 10 m.

The reason why most people don't talk about ESS is due to the fact that its presumed to take centuries or millennia for these slow feedbacks to kick in. But the issue now is that the rate of change is many times faster than any natural rate in Earth's history. Only comparable with catastrophic rare events such as the meteorite strike that took out the dinosaurs some 66 million years ago. This means that longer-term “slow” feedbacks such as melting of ice sheets and changes in permafrost carbon stores are starting to occur now, much quicker than expected, and will likely impact humanity during this century.

Which means that on top of some more warming from rapid feedbacks that has yet to be realised due to thermal inertia we also face the consequences of slow feedbacks already coming into play. These biogeophysical forces are incredibly strong and could become dominant in driving the system. Thus limiting the range of potential future trajectories.

Main point: Earth's climate is more sensitive to forcings than standard scenarios of future warming assumes

Biogeophysical Feedbacks

Some of the key negative (dampening) feedbacks such as carbon uptake by land and oceans and reflectivity by ice and snow that have maintained the Earth system in favourable conditions are weakening. We are now witnessing ever more systems close to or passing a threshold, tipping point, causing abrupt change. The challenge with tipping points is that they're often easiest to identify in retrospect.

For example, Arctic sea ice crossed a tipping point in 2007 and is now in terminal decline and could be gone during the summer by 2040 or earlier. Due to the loss of reflective ice the dark oceans are now absorbing more energy, in turn accelerating regional warming, further melting ice and snow. It also influences jet stream patterns causing more extreme weather events in northern latitudes. The loss of Arctic sea ice has also flipped the Barents Sea from acting as a buffer between the warmer Atlantic and colder Arctic ocean to now being essentially an extension of the Atlantic.

A warmer Arctic also leads to thawing of permafrost in the region. Before believed to be a rather gradual process, new studies show abrupt (decades) thaw in Alaska and Siberia due to the formation of thermokarst lakes. Releasing CO2 and CH4 to the atmosphere and accelerating warming. 

The Greenland ice sheet is now melting rapidly, the ice caps melting irreversibly. Accelerated surface melt has doubled Greenland's contribution to global sea level rise to 0.74 mm per year since 1992–2011. The interior ice sheet could cross a tipping point slightly under 2C warming. Global sea level rise has accelerated to 4.8 mm/yr

The Amundsen Sea sector of the West Antarctic Ice Sheet has already crossed a tipping point and is melting irreversibly. This will likely trigger a collapse of the rest of the West Antarctic Ice Sheet on decadal time scales. Leading to at least 1 meter sea level rise this century. Partial deglaciation of the East Antarctic ice sheet is likely for the current level of atmospheric carbon dioxide, contributing to about 5 metres of sea level rise in the first 200 years.

Melting freshwater pouring into the Atlantic has slowed down the Atlantic Meridional Overturning Circulation (AMOC) that transports heat from the Gulf of Mexico to Northern Europe. Slightly cooling northwest Europe and piling up heat along the southeast waters of the US. This in turn increases temperature differentials between tropical and sub-polar waters that can drive stronger storms. 

Main point: Abrupt changes are already occurring in the climate system, passing 2°C would likely prove catastrophic

Human feedbacks on the system

As I have explained above, the climate system is much more sensitive to even small perturbations than most people think. Another way of showing this fact is to look at human impacts on the climate before industrialisation. 

Since the rise of agriculture, human activities on Earth have played a role in shaping ecological and climatic conditions. There is good evidence to suggest that the rise of agriculture actually had a positive (amplifying) feedback on early climate, hindering a new ice age to occur. 

Atmospheric CO2 and CH4 increases during the last few millennia are anomalous compared to preceding interglacial periods. The same time period when agriculture spread across the continents and emitted greenhouse gases by clearing forests for crops and pastures, domesticating livestock and burning crop residues. Suggesting that emissions were large enough to warm climate and prolong the natural interglacial warmth.

Ruddiman et al. (2016) show evidence for what seems to be a trend brake in naturally falling CO2 and CH4 concentrations some 6000-5000 years ago, towards increasing concentrations most likely driven by anthropogenic forcing.

We know that agriculture spread across the world during this time period. Agrarian civilisations started to flourish along the Nile, Tigris, Euphrates, Indus and Yellow River some 7000-5000 years ago. Cultivation was dependent on flow and ebb cycles that in turn relied on seasonal rains and melting snows packs in the mountains. These formed the conditions for production of surplus food (energy) which allowed societies to expand and grow more complex.

Ruddiman and colleagues show how the development of irrigated rice paddies in Asia and widespread livestock domestication some 5000 years ago coincides with increases in methane emissions. Just like today, forests were cut down, vegetation slashed and burned to make way for agriculture all across Eurasia, Africa and the Americas. This generated CO2 emissions which in turn impacted climate. 

Archeological data records a shift from forest cover to more open vegetation in northern and central Europe that began som 6000-5000 years ago and was complete by the start of the industrial era. Similarly, early deforestation was likely caused around the Mediterranean by extensive land use by Greek and Roman civilisations. In Britain and France, forests had already been reduced to near-modern levels by 2500-2200 years ago.

East central China had widespread forest cover until 8000 years ago, followed by a persistent decrease especially after 6000 years ago. Archaeological sites, proxy for population density, in central China increased thirtyfold between 8000–7000 and 5000–4000 years ago. By 4000 years ago, coal had come into use as a fuel source in the Yellow River Valley because of lack of wood. Deforestation of southern China during the spread of rice agriculture after 5000 years ago added to the ongoing CO2 increase.

In India, sedentary farming and clearance emerged between 5000 and 3500 years ago, with especially rapid settlement expansion on the Deccan Plateau and in the Ganges plains. 

All this evidence provides support for the idea that large-scale deforestation led to a rise in CO2 during the middle and late Holocene. Many models have missed this because they assume low population numbers and small forest clearance per person and thus show low emissions. But this doesn't fit with historical evidence of larger per capita forest clearing 2500-1000 years ago than during industrial times. Probably because land use was inefficient and required large amounts of land but became more intensive over time as agricultural methods changed.

The simulation above indicates much greater deforestation during the millenia preceding the industrial era in agreement with pollen evidence. In contrast, standard reconstructions that assume small constant per capita clearance during preindustrial times show 40-80% of forest cover still persisting in Europe by the year 1800. Meaning massive deforestation must have taken place within the last 200 years to explain current low forest cover. But this doesn't fit with historical evidence of pervasive reforestation in western and central Europe since 1800, not deforestation. 

Main point: The Holocene climate was partly a consequence of human feedbacks on the climate system

Climate Change Adaptation

Changes in temperature and precipitation have always impacted people by affecting what they could and couldn't grow to harvest food (energy) for survival. 

The climate stabilised about 7,000-5,000 years ago coinciding with the flourishing of agrarian civilisations along the Nile, Tigris, Euphrates, Indus and Yellow River. Cultivation was dependent on flow and ebb cycles that in turn relied on seasonal rains and melting snows packs in the mountains. These formed the conditions for production of surplus food (energy) which allowed societies to grow more complex.

But agrarian societies have always been vulnerable to climatic changes. Sudden cooling events or extended droughts caused widespread famines and sometimes collapsed entire communities. Especially vulnerable were those who relied on single crops or undermined the ecological base for survival for example through intense deforestation. 

For example, a sudden cooling that happened around 3,700 to 3,000 years ago greatly influenced populations in Asia. The most dramatic changes were seen in high latitude and high-altitude areas in Mongolia and the Tibetan Plateau. Crops started to fail and widespread famine took hold. This forced people to migrate, shift to more cold resistant crops, or turn to pastoralism. Cooling temperatures also affected Northern China between AD 291-360, a time when the Chinese capital was relocated from Xian to what is now Nanjing, in the south. Again, people would have had to adapt by migrating, changing crops, herding cattle or trading. It was not an easy process and lots of conflicts arose.

The difference now is of course that the rate of change is much more rapid and that its becoming hotter, not colder, which humans have had less of an experience adapting to. Furthermore, there are no virgin lands left to move to when one region becomes uninhabitable, the world is full and most ecosystems severely degraded. Using migration as a tool for adaptation doesn't work that well anymore. We have also become heavily reliant on just a few crops and undermined diversity by eradicating species. This makes our current civilisation very vulnerable to a changing climate.

Main point: Humans can adapt to a changing climate but this time the rate of change is much more rapid and migration is not a good option


Out of the ashes into the fire

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