The Atmosphere’s Shift of State and the Origin of Extreme Weather Events

Greenhouse Gases (Photo: CECAR)

by Professor Andrew Glikson, Earth and paleo-climate scientist at Australian National University, The Conversation: theconversation.edu.au

The linear nature of global warming trends projected by the IPCC since 1990 and as late as 2007 (see Figure 1) has given the public and policy makers an impression there is plenty of time for economies to convert from carbon-emitting industries to non-polluting utilities.

Paleo-climate records suggest otherwise. They display abrupt shifts in the atmosphere/ocean/cryosphere system, as manifest in the ice core records of the last 800,000 years.

This suggests high sensitivity of the climate system to moderate changes in radiative forcing, whether triggered by changes in solar radiation energy or the thermal properties of greenhouse gases or aerosols. In some instances these shifts have happened over periods as short as centuries to decades, and even over a few years.

Figure 1: Global surface temperature rise trajectories for the 21st century under varying carbon emission scenarios portrayed by the IPCC AR4 2007. A2 represents the business-as-usual scenario consistent with currently rising global emissions. IPCC

Examples of abrupt climate shifts are the 1470 years-long Dansgaard-Oeschger intra-glacial cycles, which were triggered by solar signals amplified by ocean currents, and the “younger dryas” cold interval, which occured when interglacial peaks resulted in extensive melting of ice and cooling of large ocean regions by melt water.

The last glacial termination (when large-scale melting of ice occurred between about 18,000 to 11,000 years ago) is attributed to transient solar pulsations of 40–60 Watt/m2 affecting mid-northern latitudes. This led to a ~6.5+/-1.5 Watt/m2 rise in mean global atmospheric energy level, which meant a mean global temperature rise of ~5.0+/-1.0 degrees Celsius and sea level rise of 120 meters (see Figure 2).

Figure 2: Comparison between radiative forcing levels of (1) the Pliocene (~400 ppm CO2; T ~ 2-3 degrees C; Sea level 25+/-12 meters higher than pre-industrial); (2) the last Glacial Termination (~6.5+/-1.5 Watt/m2; ~5.0+/-1.0 degrees C; SL rise 120 meters) and (3) Anthropogenic 1750-2007 warming (1.66 Watt/m2 + 1.35 Watt/m2 – the latter currently masked by sulphur aerosols). Modified after Hansen et al 2008

As shown in Figure 2, anthropogenic carbon emission and land clearing since 1750 have raised the atmospheric energy level by +1.66 Watt/m2. Once the masking effect of industrial sulphur aerosols is taken into account. This totals ~3.0 Watt/m2, namely near half the radiative forcing associated with the last glacial termination.

Compounding the major rise in radiative forcing over the last ~260 years is the rate of greenhouse gas (GHG) rise. This has averaged ~0.5ppm CO2 per year since 1750. That’s more than 40 times the rate during the last glacial termination, which was 0.012ppm CO2 per year. The current CO2 rise rate - 2ppm a year - is the fastest recorded for the Cainozoic (the period since 65 million years ago) (see Figure 3).

Figure 3: Relations between CO2 rise rates and mean global temperature rise rates during warming periods, including the Paleocene-Eocene Thermal Maximum, Oligocene, Miocene, glacial terminations, Dansgaard-Oeschger (D-O) cycles and the post-1750 period. Glikson

We have seen this scale and rate of radiative forcing, in particular since the 1970s, expressed by intensification of the hydrological cycle, heat waves and hurricanes around the globe. It imparts a new meaning to the otherwise little-defined term, “tipping point”.

Between 1900 and 2000, the ratio of observed to expected extremes in monthly mean temperatures has risen from ~1.0 to ~3.5. From about 1970 the Power Dissipation Index (which combines storm intensity, duration, and frequency) of North Atlantic storms increased from ~1.0 to ~2.7-5.5 in accord with tropical sea surface temperatures which rose by about 1.0 degree Celsius.

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