What is Global Warming?

Global Warming: Earth on Fire

What is Global Warming C O N T E N T S Global Warming Overview Rise of Global Surface Temperature Rise of Global Sea Level The Science of Global Warming     Greenhouse Effect     Radiative Forcing Global Warming Causes & Effects Case Study: Arctic Ice Cover Shrinkage Global Warming: Conclusion

Global warming is quite a recent phenomenon that came to international prominence only by the end of the 20 th century.

But man-made pollution was known to affect the climate system of the planet as long ago as the middle – end of the 19 th century.

It was first suggested in 1863 that changes in the composition of the atmosphere due to pollution could lead to climate change. In 1896 the Swedish scientist Svante Arrhenius made first actual calculations of the effect of greenhouse warming in which he estimated that a doubling of carbon dioxide [in the atmosphere] would increase the global average temperature by 4 ° C to 6 ° C. (2)

As a general guidance, we identify the following three main indicators of global warming:

• Rise of air and ocean temperature,
• Rise of global sea level, and
• Loss of snow & ice cover

What is Global Warming:
Rise of Global Surface Temperature

Scientists estimated that, since the late 19 th century there was a real (though irregular) increase in the global surface temperature.

A trend towards global warming was observed during the period of 1910 to 1940. The temperatures declined slightly from 1940 through 1975, picking up again during the 1980s. (3)

The decline of the temperature during the post-World War II period may have been due to the masking of global warming by aerosols. (4)

During a 100-year period of 1906 – 2005, the global average temperature rose by 0.74 ° C. (5)

There is no reasonable doubt that during the last couple of decades the planet witnessed some of the hottest years on record.

As an example, 11 of the 12 years between 1995 and 2006 have ranked among the 12 warmest years since 1850. (6)

What is Global Warming:
Rise of Global Sea Level and Loss of Snow & Ice Cover

During the 20 th century, the sea levels rose by 17 cm (6.7 inches); they rose faster in the 2 nd half of this century than in the 1 st. (10)

The sea level rise is consistent with the temperature rise and general warming tendency.

It has been predicted that by the end of the 21 st century the sea level may rise by further 18 to 59 cm (7 to 23 inches), depending on a range of different scenarios. (11)

Loss of snow cover has been another indicator of climate change. Snow cover in the Northern Hemisphere declined by 4% between 1920 and 2005. (12)

As a specific example of this problem, Canada’s eastern Arctic Archipelago decreased by 15% between 1969 and 2004. (13)

The decrease in snow cover has also been consistent with the general warming trend.

What is Global Warming:
The Science of Global Warming

In order to understand the mechanics of global warming, let’s see how the Earth is “powered” by the Sun (through solar radiation), how the energy (radiation) between these two entities is continuously recycled, and how this cycle maintains the equilibrium temperature of the Earth.

Here is how this process works: (14)

• The Earth receives its original supply of shortwave radiation from the Sun
• The Earth then reflects 30% of this solar radiation back into space in its original shortwave form
• The Earth absorbs the remaining 70% of this solar energy and then re-radiates it back into space in the longwave form (infrared radiation)

So in order to maintain thermal equilibrium, the amount of shortwave and longwave radiation leaving the Earth must be equal to the amount of the original shortwave radiation received from the Sun.

Thus, for a certain amount of radiation being exchanged with the Sun the Earth will achieve a certain equilibrium temperature.

The Greenhouse Effect

Experts point out that for the Earth as a whole, the equilibrium “radiative” temperature (i.e. the temperature required to reradiate all the energy received from the Sun) at the outside of the atmosphere is −18°C. So if the Earth’s atmosphere was totally transparent to all wavelengths of radiation, then the average temperature at the surface of the planet would also be −18°C [making it most likely uninhabitable]. (15)

But thankfully, during the last step of the radiation cycle described above – when the Earth re-radiates solar energy back into space in the longwave form, the atmosphere absorbs (“traps”) some of this upward-going longwave energy.

The atmosphere then emits longwave radiation in all directions. Some of this radiation will go back down towards the Earth adding to the original shortwave solar radiation received by the planet.

This added amount of radiation raises the Earth’s equilibrium temperature [to around +15°C] and makes life as we know it possible on our planet. (16)

The Earth’s atmosphere is a unique environment which consists of a number of naturally occurring gases.

The content of the atmosphere is roughly as follows: (17)

• Nitrogen (around 78% of the total)
• Oxygen (around 20% of the total)
• Water vapor (substantial amount)
• Carbon dioxide (small amount)
• Trace gases (hydrogen, argon, helium and other gases)

It is exactly some of these gases in the atmosphere (e.g., water vapor and carbon dioxide) that trap the upward-going longwave radiation emitted by the Earth, re-emit it in all directions and thus contribute to the warming of the planet.

The gases that take part in this process are called radiatively active gases, or greenhouse gases.

Among the most important natural and man-made greenhouse gases are:

• Water vapor
• Carbon dioxide
• Methane
• Nitrous oxide
• Ozone

There are natural concentrations of greenhouse gases in the atmosphere which work to keep the planet warm.

These natural concentrations were pretty much constant during the Holocene (a period that started roughly 11,500 years ago) and before the industrial revolution. (18)

However, with the advent of the Industrial Age at the start of the 19th century, the concentrations of greenhouse gases have increased dramatically and have been attributed to human actions.

In simple terms, this means that the functioning of the modern industrial society produces additional amounts of greenhouse gases emitted in the atmosphere.

These additional amounts trap even more longwave radiation and re-radiate it back to the Earth.

This, in its turn, leads to the rise of the surface temperature and establishment of a new “equilibrium” temperature with a new given amount of radiation in the system.

Thus, human-induced increases in greenhouse gas concentrations are now so significant that they have been largely blamed for the recently observed accelerated rates of global warming.

Radiative Forcing

Heated Earth
Photo: Thiago Kunz

Following the discussion of the greenhouse effect above, it becomes clear that the rise in the average temperature of the Earth (i.e. global warming) can take place in 3 ways: (19)

• An increase in the shortwave solar radiation that enters the atmosphere at the top
• An increase in the percentage of the radiation that actually reaches the surface through the atmosphere (currently 70% of the total solar radiation), and
• An increase in the concentration of the greenhouse gases in the atmosphere

Such increases (as well as decreases) in the quantity of radiation circulating through the Earth’s atmosphere are measured in terms of “radiative forcing”.

This net change in radiation quantity may be compared to the total shortwave solar radiation of around 342 W m 2 per year that the Earth currently receives. (20)

Positive forcing (i.e. net increase in the amount of radiation) contributes to the warming of the Earth’s atmosphere. Greenhouse gases contribute to positive forcing.

Between the years 1765 and 2000 the Earth witnessed a net increase of 2.45 W m 2 in the total amount of radiation. Carbon dioxide, an extremely important agent of global warming, has contributed around 60% to this increase. (21)

This is an example of the positive forcing effect on the global temperature.

Negative forcing (i.e. net decrease in the amount of radiation) contributes to the cooling of the Earth’s atmosphere. Aerosols are an example of a climate agent that contribute to the negative forcing effect.

To summarize it all, the greenhouse effect and radiative forcing are among the most important concepts that lie at the heart of the science of global warming.

What is Global Warming:
Global Warming Causes & Effects

The main fundamental causes of global warming are two-fold:

The first one is our total dependence on the burning of fossil fuels to generate energy to power our industrialized lifestyles.

The second one is the way we use our land to produce food, timber products etc.

These economic activities are the major drivers of an increase of greenhouse gas concentrations in the Earth's atmosphere.

See the Global Warming Causes article for more detail.

The effects of global warming are indeed numerous and far-reaching.

Among other things, they include: changed climate patterns and the rise global sea level, reduction in snow cover & glacier retreat, rise in extreme weather events such as floods & droughts, loss of plant & animal diversity, reduced water supplies and decrease in agricultural productivity.

See the Global Warming Effects article for more detail.

What is Global Warming:
Case Study – Dramatic Shrinkage of the Arctic Ice Cover

Alaskan Ice Melting
Courtesy: Flickr.com

As global warming marches on and its effects unfold, the shrinkage of the Arctic ice cover that has been observed by the beginning of the 21st century may possibly be called a “poster case” for global warming.

This particular region is not only a passive recipient of the global warming phenomenon with all its effects following, it is also an active agent in bringing about further global changes as a direct result (feedback) of the warming effects it is experiencing now.

In other words, it is both a result and a cause of global warming.

The Arctic Climate Impact Assessment prepared by the Arctic Council and released in 2004 gives an excellent overview of the current state of the Arctic region from the point of view of climate change.

All the information provided below is based on the findings of this report. (22)

Here is some evidence that global warming is indeed affecting the Arctic region:

• In the past few decades the Arctic average temperature increased at twice the rate as the rest of the world

• Declining snow cover; widespread melting of glaciers and sea ice

• Rising river flows: river discharge to the ocean has increased and spring peak river flows are now occurring earlier

• Increased precipitation much of which comes in the form of rain, with the largest increases in autumn and winter

• Rising permafrost temperature leads to permafrost thawing

Some of the current and potential future effects of the Arctic shrinkage:

• Removal of the highly reflective Arctic ice cover reduces the albedo of the Earth – the ability of the planet to reflect a certain quantity of solar energy back to space. This leads to the Earth absorbing more sunlight and further contributes to climate warming.

This is exactly when the Artic ice melt becomes a cause of global warming.

• Global and regional sea level has risen by 10 – 20 centimeters during the last century, as a result of the warming process. This will affect the ocean circulation patterns and, consequently, global and regional climate

• Arctic climate change will have an impact on global biodiversity because some migratory species rely on the Arctic grounds for breeding and feeding

• Forests will replace a significant proportion of existing tundra, and tundra vegetation will move into polar deserts. The larger tree cover is expected to increase carbon removal from the atmosphere, though the reduced reflectivity of the surface will most likely outweigh this effect.

Global warming is expected to increase forest fires and destruction of trees by insects. This will lead to the loss of valuable biodiversity-rich old-growth forests.

• Reduction in sea ice will have a devastating effect on polar bears, ice-dependent seals and local people who use these animals as a food source

• Reduced sea ice will allow for extraction and shipping of oil which will further damage the marine habitat and negatively affect the health of local populations

• Many coastal communities face exposure to storms because rising sea level and a reduction in sea ice will allow high waves and storm surges to reach the shore; these storms in their turn will lead to coastal erosion

• Indigenous communities face major economic and cultural impacts whose lifestyles are closely linked to the health of their ecosystems

It is becoming more obvious that changes to the Arctic system are happening much faster than predicted even several years ago.

The WWF 2008 update report on the Arctic climate change makes a reference to ”the recent severely accelerated melting of both the Greenland Ice Sheet and the arctic sea ice.” (23)

For example, the extent of Arctic sea ice was the lowest on record in September 2007. During this period, the extent of sea ice was 39% less than its 1979 – 2000 mean. The main reason for this is believed to be climate warming which reduced both the area and thickness of multi-year ice and made the remaining ice more vulnerable to summer thaw. (24)

The melting of the Greenland Ice Sheet would have a particularly significant effect on global sea level. If this entire ice sheet melted, it would raise the sea level by 7.3 meters, with destructive effects all over the planet. (25)

It is pretty clear that the real solution to the Arctic shrinkage problem lies in solving the issue of global warming as such.

The sooner we sort it out, the better – the time is certainly running out fast.

What is Global Warming:
Conclusion

Global warming is probably the biggest challenge that humanity is facing right now.

The evidence is clear: global warming is caused by our own actions.

Up to today, we have already emitted such huge amounts of greenhouse gases that they will be enough to cause the warming to continue for decades and possibly even centuries ahead.

There is a real need for every single citizen of the Earth to act, and act right now, no matter how small our actions may seem to us.

As they say, tough times bring great opportunities.

Maybe, this period of trial for the whole planet is also a great opportunity for us to bury our differences and come and act together - there is simply no other real way to beat this global challenge.

Last update: May 2008