The current shift in state of the atmosphere/ocean system

 Andrew Glikson, Earth and Paleoclimate scientist, Australian National University, and Canberra Regional Meeting

The history of the the Earth’s atmosphere, ocean and ice shows that, once the energy level (forcing) within the atmosphere/ocean system rises beyond an upper threshold, irreversible tipping points ensue, shifting the system to a new state [1].

A shift in state of the atmosphere/ocean system would render large parts of the continents uninhabitable, lead to accelerated sea level rise and result in the extinction of many species unable to adapt to the changing environment in time. Current emission rates of about 2–3 ppm CO₂/year (Figure 1) exceed any observed in the geological record of the last 65 million years. Fossil fuel reserves (Figure 2), if continued, are capable of returning the atmosphere to tropical greenhouse conditions such as existed prior to the formation of the Antarctic ice sheet about 32 million years ago.

 

 

Climate science indicates that, since the mid-1980s, the atmosphere-ocean-cryosphere (ice) system has been accelerating toward an irreversible shift from a climate state which has favoured land cultivation since about 7000 years ago to a climate state characterised by mean global temperatures about 2–3°C above pre-industrial levels. At this level, high-intensity weather events would render large parts of the continents unsuitable for agriculture. A temperature rise of around 2–3°C implies a rise of sea level of between 5 and 40 meters (estimated for the late Pliocene [2]) during the next 100–200 years, depending on the speed at which the ice sheets melt. The following summary points and figures point to the proximity of irreversible tipping points in the atmosphere/ocean system:

 

1. Since the 19th century, the rise in the atmosphere/ocean energy level (forcing) has reached about 3 Watt/m², equivalent to a temperature of about 2.2°C above that of peak interglacial periods during the last 400,000 years (Figure 3). Approximately 1.2 Watt/m2 (equivalent to 0.8 to 1.0°C) are currently masked by industrially emitted sulphur aerosols which stay in the atmosphere for only a short time. The measured mean rise in the global land-ocean temperature since 1880 is nearing 0.9°C.

2. This rise in mean global temperature during 1880-2013 of about 0.9°C, related to a rise in atmospheric CO₂ of nearly110 ppm (Figure 1), suggests an average warming rate of 0.08°C per 1 ppm CO₂ – although such estimates do not account for the magnitude of the amplifying effects of feedback effects (from warming water, draughts, fires, and methane release). Assuming such an increase, CO₂ will reach around 472– 508 ppm by mid-century. A level of approximately 500 ppm CO₂ is regarded as the upper stability limit of the Antarctic ice sheet [3].

3. Current emission rates, fossil fuel reserves (Figure 2) and future emission trajectories are capable of returning the atmosphere to tropical Earth conditions such as existed prior to the formation of the Antarctic ice sheet about 32 million years ago. However, the impact of extreme weather events and sea level rise on large industrial and communication centers is likely to render industrial emissions a self-limiting process.

4. Retardation of ice sheet melting due to hysteresis would prevent the climate from reaching equilibrium conditions under forcing induced by 500 ppm CO₂, equivalent to a temperature rise of approximately 3°C above pre-industrial levels. However, under current emission rates a rise toward – 3°C, i.e. to Pliocene-like conditions, would proceed during the present and future centuries.

5. Pliocene sea levels have been estimated as 5 to 40 meters above recent levels, with a mean of 25 meters [4]. Such rise would inundate the world’s fertile deltas and lower river valleys where the bulk of food is produced by populations of over 2 billion people.

6. Current CO₂ rise rates of 2–3 ppm/year exceed any observed in the geological record of the last 65 million years (Figure 4), threatening to accelerate the current mass extinction of species [5].

 

The climate shifts indicated above are likely to be interrupted by abrupt fluctuations such as characterise the history of the atmosphere/ocean/ice sheets system over the last 400 thousand years (Figure 3), which have been triggered by rises in energy forcings at rates which are lower by an order of magnitude than the current rise rates of greenhouse gas forcing (Figure 4). Terminations of the ice ages (Figure 3) are attributed to a 3.5 Watt/m² increase in solar insolation, reinforced by approximately 3 Watt/m² due to feedbacks by greenhouse gases released from warming oceans and drying vegetation, a combined6.5 Watt/m² [6]. Similar solar forcing amplified by ocean current changes is invoked in connection with the sharp D-O (Dansgaard-Oeschger) approximately1500 years-long cycles during the last glaciation. The current rise in greenhouse gases since the 18th century has reached a level higher than 3.1 Watt/m^2, namely about half that of the last glacial termination (Figure 3).

There appears to be no alternative to a global effort at deep cuts of carbon emissions coupled with fast-tracked CO₂ sequestration, including large-scale application of reforestation, biochar and chemical CO₂ down-draw methods. In Australia, the combustion and export of hundreds of million tons of carbon as coal, coal seam gas and natural gas [7] renders decisions in this country of critical importance for the future.

As stated by Professor Joachim Schellnhuber, Germany’s climate advisor and Director of the Potsdam Climate Impacts Institute:

“We’re simply talking about the very life support system of this planet”.