Global Warming

I

Introduction

Global Warming, term denoting the accelerated warming of the Earth’s surface due to anthropogenic (human activity-related) releases of greenhouse gases due to industrial activity and deforestation.

II

Earth’s Energy Balance

But for the greenhouse effect, life on Earth would not exist. The Sun emits radiation to the Earth. If we could imagine a flat surface at the top of the atmosphere, that radiation is about 340 watts per square metre (340 W/m^-2). Just over 100 W/m^-2 is reflected out again by atmospheric aerosols and clouds, and the Earth’s surface, leaving some 240 W/m^-2 that heats up the surface of the Earth. The system must be in balance—energy “in” must equal energy “out”—so the Earth needs to re-radiate this amount back into the atmosphere. But the amount actually re-radiated depends on the Earth’s surface temperature: the hotter the surface is the more it will emit radiation. The outgoing radiation takes the form of “long wave” infrared thermal radiation. If the system balanced “naturally”, then the Earth’s surface would have a temperature of about –19° C (-66° F) since at this temperature 240 W/m^-2 would be emitted. Obviously, something else must be happening because at such low average temperatures life would not exist. The Earth’s surface is very much warmer than this “natural” level (around 15° C/59° F) and hence far more radiation is emitted than the 240 W/m^-2. What happens is that a lot of the Earth’s re-radiation bounces back to the Earth’s surface because it gets absorbed mainly by water vapour and carbon dioxide (CO₂) in the atmosphere. Water vapour, CO₂, and a few other minor gases act like a “blanket”. The balance is secured as follows:

• Incoming solar radiation: + 340 W m^-2
• Reflected from clouds, the Earth’s surface, etc.: - 100 W m^-2
• Net incoming radiation absorbed by the Earth = + 240 W m^-2

• Outgoing radiation: - 420 W m^-2
• Greenhouse effect: + 180 W m^-2
• Net outgoing (thermal) radiation = - 240 W m^-2

The way the system balances, then, is that the Earth’s surface warms up compared to what would happen if the Earth was not surrounded by a blanket of greenhouse gases.

III

Anthropogenic Greenhouse Gases

So far nothing is amiss. Indeed, the greenhouse effect is a good thing for life on Earth. The problem arises because humankind is adding to the effect by increasing the amounts of CO₂ and a few other gases in the atmosphere, notably methane (CH₄) and nitrous oxide (N₂O). This results in the enhanced greenhouse effect, or “global warming”. Since the concentration of water vapour tends to be fixed (it is determined by the oceans) imagine what would happen if the atmospheric concentrations of CO₂ were increased. The effect would be to increase the radiation bouncing back to the Earth and reducing the radiation leaving the top of the atmosphere. For a doubling in CO₂ concentrations, the reducing atmospheric radiation would be about 4 W/m^-2. But the system is now out of balance: 240 W/m^-2 is coming in but 236 W/m^-2 (240 W/m^-2 minus 4 W/m^-2) is going out. In order to balance, something must change, and what changes is the temperature of the Earth’s surface. Recall that if it increases, outward radiation will increase. This will happen until the 240:240 balance is restored. But while the balance is restored, the Earth has basically got hotter. For each doubling of CO₂ concentration, the temperature increase is expected to be about 1.2° C. Various complicating factors intervene to enhance or reduce this figure. Water vapour might increase and this would make the enhanced greenhouse effect stronger still. Other factors of relevance are changes in cloud formation, changes in surface vegetation, the melting of the tundra (which would release methane), changes in ocean circulation, the cooling effects of sulphur aerosols, and so on. The end result is some uncertainty about projected climate change but an average temperature change of about 2° C by 2100 might be expected.

Where do the greenhouse gases come from? The fact that they come from economic activities that are so pervasive to human society largely explains why global warming control is so complicated and so controversial. CO₂ is emitted from the burning of fossil fuels, so that most electricity production and most industrial activity contribute to global warming. Since gasoline, kerosene, and diesel are fossil fuels, they too contribute, which means that the entire transport sector is implicated. Methane is also emitted from fossil fuel burning, but also from gas pipeline leaks and from decomposing vegetation. Methane emissions are therefore associated with livestock and with rice growing. Nitrous oxide comes from fossil burning and fertilizers. The burning of forests also contributes significantly to CO₂ emissions.

IV

The Impact of Global Warming

The next issue is to predict what would happen if these temperature changes were allowed to happen. The science of climate change impact assessment is very uncertain, not least because humans have the capacity to adapt to some of the expected changes. There are two stages to impact assessment: predicting what the consequences will be for ecosystem change and human health, and assessing how important those changes will be. The context to all this assessment is uncertainty, not least because the rate of change of temperature and the levels of temperature change together place some of the change outside human experience. That is, we have little idea how environments and humans will respond if the worst-case scenarios occur. An additional complication is that impacts will vary region by region, not just because of different susceptibilities but because there will be regional variations in temperature change, in precipitation, and in extreme events such as hurricanes. Summer monsoons in Asia could become heavier, but summer rains in southern Europe could become less.

The kinds of impacts that would seem to be important are as follows. Sea levels will rise due to the thermal expansion of the oceans. Low-lying areas, such as the coastal regions of Bangladesh, and many small islands, could be seriously affected unless adequate sea defences are built and maintained. Fresh water resources could be affected by saline intrusion as sea levels change. Existing dry land regions may become drier still, resulting in a greater likelihood of desertification. Agricultural output may change adversely in some regions, due to reduced rainfall, but may increase in other areas because CO₂ also has a “fertilizing” effect on crops. While most of the work on impacts has been carried out on the agricultural sector, it is not clear that world food supply will be significantly affected: some regions will lose and some will gain. But the regions suffering losses may be some of the poorest in the world. In terms of human health there are similar ambivalent effects: if winter temperatures rise there may be fewer premature deaths due to winter cold. But if summer temperatures also rise there may be added deaths from heat stress. The pattern of the world’s diseases may also change—diseases such as malaria, eradicated from Europe, could return to some areas. Perhaps the most important effects are the ones we know least about. Ecosystems change in response to climate change but, in general, past changes have occurred slowly as temperatures varied over long periods. A rise of 1 or 2° C in just a century is a very fast rate of temperature change, and some ecosystems may not be able to adjust. Even more speculative are the effects of extreme events: for example, the worsening of El Niño, and the potential effects on ocean currents and hence marine productivity.