First point – the global climate is changing. Not many people dispute that any more. The mean global temperature has risen by 0.8°C over the past century, and the ten warmest years on record have all occurred since 1998. Within the past century many significant climate changes have been measured and reported, including increases in the frequency of heat waves in the U.S., an increasing proportion of precipitation coming in the form of intense, flood-inducing events, an increase in tropical cyclone intensity in the Atlantic Ocean, Caribbean, and Gulf of Mexico, a huge decrease in the seasonal extent of Arctic sea ice, and a big jump in the rate at which glaciers are melting.
The rates of change seem to be accelerating and most of the profound secondary changes are negative. Dr James Hansen, the NASA scientist who first drew international attention to the impending climate disaster, testified way back in 1988 that Earth had entered a long-term warming trend. Today the effects of global warming on the extremes of the global water cycle – stronger droughts and forest fires on the one hand, and heavier rains and floods on the other – have become more evident in Australia, Europe, North America, Africa and Asia.
Second point – the causal factors of climate change are now very well known. Earth is surrounded by a relatively thin layer of greenhouse gases – water vapour, carbon dioxide (CO2), methane and nitrous oxide – which act as a thermal blanket. About half the incoming solar radiation passes through the atmosphere to the Earth’s surface where some is absorbed and the remainder reflected back into the atmosphere. Substantial amounts of the energy absorbed are again radiated outward in the form of infrared heat. These contribute further to the warming of the atmosphere.
Third point – humanity has drastically changed global climatic dynamics by adding huge amounts of CO2, methane, nitrous oxide and chlorofluorocarbons to the atmosphere. Activities such as deforestation, land use changes and the burning of fossil fuels have increased atmospheric CO2 by a third since the Industrial Revolution began. Decomposition of wastes in landfills, burgeoning agriculture, especially rice cultivation, and huge populations of burping and manure-producing domestic livestock have boosted the amounts of methane in the atmosphere by a factor of three since the industrial revolution. Methane is twenty times more active than CO2 in atmospheric heat retention.
The atmospheric concentration of CO2 measured at the Mauna Loa Observatory in Hawaii is a good indicator of where we are now globally in respect of atmospheric change. Back in 1959 when the data collection programme was initiated by the National Oceanic and Atmospheric Administration (NOAA) the CO2 level was measured at 316 parts per million (ppm) and the annual increase was less than 1 ppm. Today the level is over 392 ppm and the annual increases are 2.2 ppm and getting larger all the time.
James Hansen and his climate scientist colleagues concluded that we have either reached, or are very close to, a set of climate “tipping points”. That means that climatic changes are now at a point where the feedbacks from changes spur even larger and more rapid further changes. Hansen cites Arctic sea ice as a good example of this. Global warming has initiated faster sea ice melt and has exposed darker ocean surfaces that absorb more sunlight which leads to more melting of ice. As a result, and without any additional greenhouse gases, the Arctic could soon be ice-free in the summer. The western Antarctic and Greenland ice sheets are vulnerable to even small additional warming – once disintegration gets well under way it will become unstoppable.
Pause for reality check – not only is climatic change a reality, it is progressing at an accelerating rate, the negative consequence are getting greater, and the likelihood of us managing to slow or reverse the negative trends are getting smaller.
Fourth point – James Hansen and his fellow climate scientists looked at the atmospheric CO2 levels, then at the changes in climate which were occurring, and came up with the recommendation that a CO2 level of 350 ppm (last recorded back in 1987) was pretty much the upper allowable limit if massive climatic related adverse effects were to be avoided. The number 350 has a certain appealing ring to it, and has been widely adapted by environmental organizations such as Bill McKibben’s 350.org as a universal target for citizen and government action on carbon emissions. The protagonists are quite aware that the present global atmospheric CO2 level has already overshot that target by more than 40 ppm, but they argue, convincingly, that a reversal is absolutely essential to safeguard our long-term global future.
Fifth point – and now we’re at the crux of the problem. How on Earth, or anywhere else for that matter, do we get anywhere close to reducing the rate at which atmospheric CO2 increases in future, never mind actually reversing the trend towards 350 ppm?
We think of Earth’s carbon reservoirs as being great fields of coal and petroleum compounds, which are more or less stable until we dig them up and burn them. But the globe’s biggest carbon reservoirs are in the atmosphere, the ocean, living ecosystems and soils, and are highly dynamic. They all exchange CO2 with the atmosphere, they both absorb it (oceans) and assimilate it (ecosystems), and they release it (oceans) or respire it (ecosystems). The critical point is that anthropogenic carbon emitted into the atmosphere is not destroyed but adds to the stockpile and is redistributed among the other carbon reservoirs. The turnover times range from years or decades (living plants) to millennia (the deep sea, soil). The bottom line is that any carbon released into the atmosphere is going to be around for a long, long time. Up to 1000 years in fact.
Sixth point – so how do we get from our present scene of 390 ppm CO2 in the atmosphere and impending climate doom to something closer to 350 ppm and a more stable climate scenario? Straight answer – we cannot. We simply don’t have that option.
Seventh point – the absolutely best case scenario for reduction of CO2 emissions to the atmosphere would be an immediate halt to all activities leading to anthropogenic carbon emissions. Park all motor vehicles, no more home heating, no coal-fired power plants, no burning of natural gas, no aircraft flying overhead, shoot and bury 90% of all domestic livestock. Just shut down all of human civilization. No more anthropogenic carbon emissions. Would this sacrifice bring the CO2 level down in a hurry?
Dr Susan Solomon and her colleagues at NOAA, with the help of their sophisticate computer models have addressed that very question. They ran a coupled climate–carbon cycle model which has components representing the dynamic ocean, the atmospheric energy–moisture interaction, and interactive sub-models of marine and terrestrial carbon cycles. The model reveals, sadly for us, that climate change is largely irreversible for 1000 years after all carbon emissions cease. The drop in radiative forcing of atmospheric CO2 (i.e. the extent to which CO2 causes atmospheric warming) is largely compensated by slower loss of heat to the oceans. So atmospheric temperatures do not drop significantly for at least 1,000 years. And the natural interactive processes between the atmosphere, ocean and ecosystems would carry on. Atmospheric CO2 concentration would eventually drop back to 350 ppm by about 2060 and then flatten out to near 300 ppm for the rest of the 1000 years.
Eighth point – I haven’t noticed any great urges on the part of ourselves to go and huddle in caves and gnaw on pine nuts and raw fish (no wood-burning allowed) to make this scenario work, so what is more likely?
Global carbon emissions from fossil fuel use were 6.2 billion tonnes back in 1990 when global CO2 was near 355 ppm. The 2010 estimate is 8.5 billion tonnes. That’s a 38 % increase over the levels used to formulate the Kyoto Agreement. The annual growth rate of emissions derived from fossil fuels is now about 3.5%, an almost four-fold increase from the 0.9% per year for the 1990-1999 period. Carbon emissions from land-use change (i.e. mainly deforestation) in 2007 (in just that one year) were estimated at 1.5 billion tonnes of carbon. The biggest increase in emissions has taken place in developing countries, largely in China and India, while developed countries have been growing slower. The largest regional shift has been that China passed the U.S. in 2006 to become the largest CO2 emitter, and India will soon overtake Russia to become the third largest emitter. Currently, more than half of the global emissions come from less developed countries. Developing countries with 80% of the world’s population still account for only 20% of the cumulative emissions since 1751. There is nowhere for these rates to go, other than up.
When the Intergovernmental Panel on Climate Change produced their Fourth Assessment Report in 2007, they diplomatically tried to hedge their bets. So they churned out 40 different scenarios based on emissions scenarios for the decade 2000-2010 which encompassed the full range of uncertainties related to future carbon emissions, demographic, social and economic inputs and possible future technological developments. The model predictions were correspondingly wide, ranging from “best” to “worst” in terms of atmospheric CO2 levels and changes in the associated climatic driving forces. Now it has become apparent that the actual emissions growth rate for 2000-2007 has exceeded the highest forecasted growth rates for 2000-2010 in their emissions scenarios.
Ninth point – so the most likely future outcomes (by the end of the century) are those at the top end of the scale outputted by the computer models (diagram above). That is to say our grandchildren will be looking at CO2 levels above 900 ppm, mean global temperature rises of 5 or 6 degrees C over what they are today, and an average sea level rise above 0.5 metres. Plus all the storms, cyclones, droughts, floods, vanishing shorelines, water wars and famines that might creep in along the way.
The end – CO2 concentrations in the atmosphere and future temperatures are just numbers, and pretty much the only things that computer models can output. We will have to estimate the extent of global human misery by ourselves.