A Four Billion Year Old Heating Account

A FRAGILE FILM SEPARATES THE EARTH FROM OUTER SPACE

Earth is shrouded in a thin film of water and air, which collectively make up the oceans and the atmosphere. “Climate” describes the processes that take place within this thin film. If we reduced the Earth to the size of a grapefruit, the atmosphere would only be one millimetre thick, as would the Earth’s cold, brittle outer shell. Just about everything of any consequence to us takes place in this narrow zone sandwiched between the cold of outer space and the red-hot interior of the Earth. The atmosphere gets thinner the higher up we go and, as a consequence, 80% of the air lies below an altitude of 12 kilometres. This is the zone in which rain, hail and snow form and where the major winds blow. This important part of the atmosphere would only make up 0.1 mm of our model Earth and the oceans would, on average, be 0.03 mm thick. So, even if the sky seems high, our atmosphere and oceans are only a thin, fragile yet precious film that separates the Earth from cold, empty outer space.

THE EARTH'S CYCLES

The fact that such a small fraction of our planet is occupied by the atmosphere and oceans means that their composition and dynamics are, to a large extent, determined by the underlying Earth. Water and gases in the air become trapped in minerals and are carried to the Earth’s interior by geologic processes, then released back into the atmosphere through volcanoes and hot springs. These processes are very slow and allow geological control over the atmosphere and the oceans over very long time-spans. All the water of the oceans is recycled through Earth’s interior every 100 million years.

The CO2 of the atmosphere enters similar geologic cycles, moving between the atmosphere, the ocean and the solid Earth within time-scales of thousands to millions of years. However, the components of the atmosphere and oceans also engage in biological processes, as when we inhale oxygen and exhale CO2. During photosynthesis, CO2 is captured in organic matter, which later decomposes, returning CO2 to the atmosphere. The biological processes are generally much faster than the geological processes and this biologic carbon cycle is a thousand times faster than the geologic carbon cycle. Biology can therefore affect the composition of the atmosphere and oceans during very short time-scales.

The role of man in these bio-geochemical cycles is that we have figured out how to short-circuit the geologic carbon cycle and release the energy stored in geologic deposits of coal, oil and gas while emitting CO2 directly to the atmosphere. Our CO2 emissions follow biologic time-scales, while the removal of CO2 from the atmosphere and oceans still has to obey geologic time-scales. Therefore, the burning of fossil fuels at a rate that is out of step with the geologic mechanisms for CO2-fixation has led to a dramatic increase in the concentration of CO2 in the atmosphere.

THE NECESSARY GREENHOUSE EFFECT

Despite the high temperatures in the Earth’s interior, only a small fraction of this heat reaches the surface, because rocks are very poor conductors of heat. The temperatures in the depths are not a consequence of a high production of heat, therefore, but are caused by the fact that the miniscule amount of heat that is produced cannot get to the surface through Earth’s solid outer shell. Earth’s surface is therefore primarily heated by the sun. The sunlight that reaches one square metre of ground provides as much heat in one hour as the amount that reaches the same square metre from Earth’s interior in a full year.

About a third of the light that shines on Earth is reflected back to space and the rest is absorbed and converted to heat at the surface, in the atmosphere and in the oceans. All warm objects – including planets – radiate heat to their surroundings. The warmer the object is, the more heat it radiates out. This means that there will be a balance between the amount of light radiated on Earth and the amount of outgoing heat radiation lost to space. There is a simple physical law that describes the temperature at which this radiation balance is achieved. For Earth, this temperature is -18°C. For those who have decided not to believe in the greenhouse effect (which is a physical phenomenon not subject to belief – like gravity, it exists whether we believe it or not), it may be sobering to know that Earth would be deep-frozen without it.

The average temperature at Earth’s surface is 15°C, which is 33 degrees warmer than predicted by radiation balance alone. As early as 1896, the Swedish chemist Svante Arrhenius showed that this higher temperature is caused by trace amounts of some gases that limit the escape of heat radiation from Earth’s surface. He called the phenomenon “the greenhouse effect” and showed that, for a large part, it is caused by a very small amount of CO2 – just about 0.03 %.

Our atmosphere has a composition that lets the sunlight pass uninterrupted onto the Earth’s surface, where most of it is converted to heat. The atmosphere is not quite so transparent to heat radiation, however, because CO2, water vapour and methane absorb the heat and keep it in the lower part of the atmosphere. This means that the equilibrium temperature of  -18 °C predicted by radiation balance is not to be found at the surface but at a few kilometres’ altitude, where the air is thin enough to let the heat radiation pass through.  The atmosphere below this level is like a warming comforter that blankets the Earth’s surface from cold outer space. The greenhouse effect is therefore a requirement for life on Earth.

A hundred years ago, Arrhenius was already aware of the effects of burning fossil fuels (coal, at that time). He suggested that increasing industrialisation would lead to higher concentrations of CO2 in the atmosphere and, consequently, to higher temperatures in the future. This relationship between the CO2 content of the atmosphere and Earth’s surface temperature has thus been well understood for more than a century.

THE OCEANS AND THE EARTH'S CLIMATE

As we all know, climate has many more facets than just temperature. In addition to the heating of the atmosphere, many other processes consume the heat that is generated at the surface. A large proportion of the heat goes to heating the oceans. The mass of the oceans is more than 260 times the mass of the atmosphere and they can therefore absorb much more heat than the air. It takes four times more heat to increase the temperature of one kilogram of water than it takes for one kilogram of air. So, it takes a thousand times more heat to increase the temperature of the oceans by one degree than it does to increase the temperature of the atmosphere by one degree. A temperature increase in the oceans’ uppermost couple of metres is equivalent to an increase in the temperature of the entire atmosphere by the same amount. The oceans transport heat from the sunny tropics to the cold regions in the north and south, and they can accumulate and store heat for decades, or even centuries, before they release it again at the surface.

For this reason, we cannot expect to see a direct match between the CO2 content of the atmosphere and surface temperatures. Over long time-spans there will be a clear correlation but, on a year-to-year basis, the effects of ocean currents, winds and clouds will vastly overshadow the detailed effects of small changes in the concentrations of greenhouse gases in the atmosphere. Therefore, we should not discredit the greenhouse effect just because we observe that, even though we live in a time with steadily increasing greenhouse gas emissions, some years are markedly colder than the previous ones.

MAN'S IMPACT ON THE EARTH'S SYSTEM

Some might ask whether changes in the greenhouse effect are not a natural part of the Earth’s dynamics. From the geological record, it is clear that Earth has enjoyed a good and efficient greenhouse atmosphere for more than four billion years. There is no doubt that the concentration of CO2 has varied greatly throughout the Earth’s history and, at times, has been much higher than its present level. However, there is also no doubt that humans were not subject to these conditions, and there was no requirement for maintaining complex societies capable of supporting billions of people in just about every corner of the planet. Few scientists, if any, doubt that the CO2 content of the atmosphere has varied in concert with the coming and passing of ice ages during the past 250,000 years when humans have roamed the Earth. During these times, CO2 has varied within a relatively narrow range of concentrations. Since the beginning of the 19th century, we have observed a rapid and accelerated increase in CO2 in the atmosphere.

The added CO2 could be uniquely attributed to the burning of fossil fuels, because we can recognise a characteristic isotopic composition of carbon from this source in the CO2 in the atmosphere. This observation rules out volcanic sources of the extra CO2. There is therefore absolutely no question that the increase in CO2 in the atmosphere is caused by human activities, and that this human contribution has raised the CO2 content to a higher level than has ever been reached by natural fluctuation during the past 800,000 years. The rate of increase, at present, is 200 times higher than has ever been measured in the part of Earth’s history to which we have access.

The fact that 2008 was “the coldest year of the millennium” has caused some public confusion, and it has been loudly speculated that this might disprove CO2-induced global warming. The millennium is only eight years old, however, and 2008 was also one of the ten warmest years ever recorded. It was thus warmer than 98 of the years in the last century, and probably warmer than 998 years out of the past millennium. The statement about the warmest year in the millennium could thus easily be seen as demagogy.

Climate is the synthesis of a great multitude of factors and it is most likely that a variety of natural causes contribute to the climate changes we are observing at present. However, the flow of energy to and from Earth has been monitored by satellites for decades, and it is quite clear that there is no increase in energy contribution from the sun. Although other effects in addition to changes in greenhouse gas concentrations cannot be discounted, the increase in these gases remains the main reason for increased surface temperatures. More importantly, the effects of greenhouse gases are not cancelled out just because additional effects are discovered. If it turns out that there are some natural causes that increase the temperature of our planet, it is even more vital that we use the only “dial” that humans can control – our own emissions of greenhouse gases.

CLIMATE - GETTING BETTER OR WORSE?

So, when will disaster strike? It is not really possible to apply concepts such as “good” or “bad” to climate. Any culture will regard changes that have led to the emergence of their particular – and particularly great – society as climatic improvements. In contrast, the same climate changes will have been perceived as a climatic crisis by the preceding culture that collapsed as a result. At any given time, however, rapid change in the environment subjects life and human societies to stress.

In our current situation, it is the rate of climate change that is the root of the problem. When our conditions change at a faster rate than we can comfortably adapt to, our societies will be drained of resources because we will have to spend most or all of our efforts on adaptation. Due to the high population density of Earth today, we can no longer just move when the climate in our region challenges our way of life. We may not necessarily experience distinct climate disasters at first hand, but we will certainly experience increasingly cumbersome and troublesome daily lives in societies that are losing more and more of their defining values.