The rise of atmospheric energy (heat) level due to emission of greenhouse gases and land clearing has reached 3.1 Watt/m2, potentially leading to warming by 2.3 degrees Celsius, currently mitigated by the transient effect of sulphur aerosols and the cooling effect of the oceans.
The Earth energy balance is defined as the difference between energy (heat) absorbed by the surface from solar radiation and the energy (heat) emitted back to space. The energy rise since 1880, estimated at +3.1 Watt/m2, equivalent to a +2.3 degrees C (Hansen et al., 2011) (Figure 1) is currently mitigated to a large extent by the cooling effect of sulphur aerosols (~ -1.6 Watt/m2, i.e. -1.2C) emitted from fossil fuels, which effectively has a global geo-engineering effect (Figure 1). The atmospheric residence time of sulphur aerosols is no longer than a few years. Had it not been for the cooling effect of these aerosols the internationally agreed maximum temperature target of <2 degrees C would be transcended.
According to the IPCC AR4 (2007) mean global land/ocean temperature since 1880 has risen by about +0.8C which, as shown in Figure 2, translates to more than +4C rise in the polar region of northern Canada, Greenland and Siberia. The warming triggers amplified ice melting which has accelerated between 2002 and 2010 (Figure 3), contributing to sea level rise which reached a rate of 3.0+/-0.4 mm/year between 1993 and 2010 (Figure 4). The disappearance of sea ice from large parts of the Arctic Ocean leads to increased evaporation and to the advance of cold moist fronts into the north Atlantic (Figure 2b) with consequent snow storms.
The instrument measured rise in global warming by about +0.8C since the early 20th century is lagging behind the +1.1C rise indicated by the global energy imbalance (Figure 1B). This lag is to a large extent attributable to the buffering effect of the oceans. The temperature rise lag period is not clear but has been estimated as 35 years.
Prior to the 2009 Copenhagen conference the European Union has obtained the agreement of relevant partners, including China, India, Russia and the United States, to commit to an upper two-degree Celsius ceiling on further rise in mean global temperature. However, a rise to 2C may lead to tipping points in the atmosphere/ocean system.
Current CO2 levels have risen to 393 ppm (Figure 6). The relation between atmospheric CO2 levels and global warming is stated by the NASA/GISS climate science group in the following terms:
“Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, large scale glaciation occurring when CO2 fell to 425±75 ppm, a level that will be exceeded within decades, barring prompt policy changes. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm. The largest uncertainty in the target arises from possible changes of non-CO2 forcings. An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.”
With CO2 emissions rising to ~28,000 million tons per year by 2010 (Figure 5), increasing atmospheric levels by ~2 ppm/year, with a total CO2-e (which includes CO2 + the equivalent effect of methane) of >460 ppm, the NASA/GISS study implies global warming committed to reach 2.3 degrees Celsius once the aerosol effect dissipates (Figure 1). This effective, though originally unintended, geoengineering measure, acting as a transient shield from a near-doubling of global warming, further lowers ocean pH on top of CO2 sequestration.
Had the proposed Australian reduction of 5 percent in the rate of carbon emissions by 2020 relative to 2000 been adopted world-side, global emissions would be reduced from ~24,000 million tons/year to 22,500 million tons/year, hardly causing a dent in the current trajectory toward levels at which the polar ice sheets are further destabilized (Figure 3).
The increased level of energy in the atmosphere/ocean system is resulting in a series of extreme weather events around the globe (Figure 7). It is not clear whether deep reduction in carbon emissions will be sufficient to stem the amplifying feedbacks associated with greenhouse gas warming and ice/melt water interactions. Barring an indefinite maintenance of sulphur aerosol emissions, deep emission cuts need to be accompanied by atmospheric CO2 draw-down by means of fast-track tree planting, application of biochar methods and chemical CO2 sequestration.
The alternative bears no contemplation.
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