The Basic Science
We all know what a greenhouse is for ... a greenhouse allows light in but won't allow heat out. As the light enters the greenhouse, it strikes the plants, the ground, the tables, etc. and is converted into heat. The heat, which is really just light at a lower frequency, does not penetrate the glass walls of the greenhouse as easily as the light does, so most of the heat energy is reflected back into the house. Eventually, equilibrium is achieved where the amount of energy entering the house is equal to the radiated heat energy leaving the house ... but this equilibrium is not reached until the whole structure has become much hotter. That's the beauty of a greenhouse ... it allows us to keep the heat in.
The magic ingredient of the greenhouse are the glass walls ... it is the walls, which are transparent to light at one wavelength, but opaque to light at the lower infrared frequencies, that makes the greenhouse work.
In the same way, our atmosphere acts like glass walls, which allow sunlight in but is opaque to infrared light, which is heat. Some of the sun's light bounces straight back into space off of the clouds or off the surface of the earth ... this bouncing back is called the albedo effect. Brighter, whiter objects like clouds, deserts and ice reflect most of the sun's light before it can become heat. However, about half of the sun's energy is absorbed by the earth's surface where, like the plants and soil in a greenhouse, it is converted into heat. The darker areas of the earth are really good at absorbing the light, places like the oceans or forests. The sunlight is converted into heat energy that is then is emitted back into the atmosphere, where some of it escapes but most of it bounces around, off clouds and buildings, and returns back to the surface where it helps to maintain the surface temperature of our planet.
Just like in our greenhouse example, eventually equilibrium is reached where the amount of sunlight energy coming into the earth's atmosphere is balanced by the amount of heat energy leaving the planet.
Now, having some greenhouse gases is very good. Without any GHGs, our average temperature would be around -18C! Mars, for example, has very little atmosphere, even though what it does have is mostly CO2 ... there is just too little CO2 to create much of a greenhouse effect there. The average air pressure on Mars is less than 1 kilopascal (600 pascals to be more accurate) compared to our average of about 101 kilopascals. As a result, the average temperature on Mars is as cold as our arctic winters: -55C!
Venus on the other hand has a very healthy atmosphere ... similar to the earth in size (barometric pressure on Venus is around 92 kilopascals). However Venus' atmosphere is almost all CO2, the results of a run-away greenhouse effect that didn't stop until the equilibrium temperature level was reached at 467C. This is above the melting point of zinc, tin, and lead. Devilishly hot, you might say ... and this particular little devil is warning us not to let this happen to our lovely planet.
There are a lot of gases that can create a greenhouse effect, but the ones shown here are the ones scientists are most concerned about. These five gases have driven 97% of the global warming we have seen so far. CO2 has had the biggest impact, responsible for 2/3rds of the changes in temperature so far, and methane is next at 1/6th. Methane is actually 23 times stronger than CO2 but as you can see from the y-axis here, there is much less of it. We currently have around 385 ppm of CO2 in our atmosphere (let's call it .038%) versus about 1750 ppb for methane, which is over 100 times less than CO2. Nitrous Oxide is actually 300 times more potent as a GHG than CO2, but again, there is not so much of it up there: only around 314 parts per billion, so about a 1000 times less than CO2. In total however, all these gases are contributing to the greenhouse effect.
Oxygen and Nitrogen and a little bit of Argon, which make up 99.9% of the dry atmosphere, do not have any greenhouse effects. So all the warming that we experience on our planet, the fact that we are 32C warmer than we would be without these gases, is due to this small component of our air. Less than .1% is not a lot, so imagine what would happen if we make a significant increase in that less than .1%! We will come back to this later.
Mankind is emitting other gases that are of concern, and Mother Nature also emits some herself, but CO2 and Methane are doing the majority of the damage to our climate.These graphs, from the UN IPCC, show the changes over the last 30 years. As you can see, the trends are not good. Methane seemed to have peaked; however CO2, the main driver, is still, inexorably, moving upwards.
Although we don't emit water vapour into the atmosphere, water is a powerful greenhouse gas and does make up about .4% of our atmosphere, more than CO. Water is one of those positive feedback mechanisms that scientists worry a lot about. As we warm up, thanks to all that CO2 we are emitting, water evaporates faster, which means we warm up faster, which means more water vapour enters the atmosphere, causing the temperature to rise even more, of course which causes more evaporation and more water vapour. Thus the feedback loop starts to spiral dangerously out of control.
A question that scientist next address is - what are the relative impacts of these greenhouse gases compared to other causes that could force our climate to get hotter?
What do you think would happen if the sun started radiating more energy into our greenhouse? It would force the temperature to rise. What do you think would happen if we add more glass to the walls, and double pane the windows? Again, it would force the temperature to rise. Scientists call these "radiative forcings" and they can measure them.
The factors than can influence climate are often evaluated in terms of their "radiative forcings". This is a technical sounding term but it simply refers to the impact of the factor upon the heating or cooling of the earth. As you can see here, CO2 is again the major factor affecting the climate. The values are in watts/meter squared, just as we have seen earlier. This value represents how much extra energy the factor is holding in, or how much less it is emitting away. All the other greenhouse gases combined don't add up to the impact CO2 is having, but they do contribute a significant amount to the total.
Ozone also affects the climate, depending upon where it is. If we have more ozone in the upper atmosphere, the stratosphere, that will act to cool the planet. However, ozone down lower, in the troposphere or near the surface of the earth, causes warming. As you can see we are emitting a lot of low lying ozone, also known as smog, and thus again heating up the planet.
I mentioned that we do not contribute water vapour directly; this is not quite true. We do emit some water vapour via high flying airplanes and their contrails. This greenhouse gas is trapping heat in the stratosphere.
Next we have some good news: thanks to our deforestations we have removed dark coloured canopies from the surface of the earth and turned the land into lighter colour fields and deserts. This change in land surface, via our use of the land, makes the land less absorbing and more reflecting of sunlight. While that is a good thing - no one is suggesting we should cut down all the trees and make the whole world a desert just to increase the world's albedo.
Some other, accidental, good news: our pollutants also cause the earth to cool. It is a phenomenon called global dimming. In much the same way that the whole world cooled when Mount Pinatubo erupted in 1991, the particles we put into the air blocks the sunlight from getting in - it is like painting the glass walls of a greenhouse, which would limit how much light enters. This is called the direct effect - but, here is a bit of a paradox, as the world cleans up our air, as we are doing by burning less coal and more, cleaner natural gas, we are actually reducing this dimming and allowing more energy into our system. The other effect shown here is another accidental side benefit of our pollution: with more aerosols and particulates in the atmosphere, clouds form more easily. Clouds being nice and white, reflect sunlight back out into space. In this respect, water vapour forming clouds are good. Unfortunately, not all water vapour that global warming creates becomes clouds. [There is a lot of water vapour in the room right now, but we can't see it because it hasn't condensed into a fog or a cloud.]
Here is an excerpt from Science on how aerosols affect the climate. "Some aerosols, such as sulphates, have a cooling effect on climate because they reflect solar radiation, while other aerosols, such as black carbon, have a warming effect because they absorb radiation. The preponderance of reflective aerosols in the atmosphere has caused a net cooling effect on climate, offsetting the warming caused by carbon dioxide by almost a third according to an estimate by the IPCC. However, the amount of cooling is highly uncertain because estimates based on global aerosol models and observation-based estimates differ widely. In a Report published online in Science Express, G. Myhre used a combination of observational data and modeling to reconcile the two approaches. The study finds that cooling from the direct effect of aerosols is about 40% less than the IPCC estimate (0.3 Watts per square meter rather than 0.5 Watts per square meter). The reason for the discrepancy is that the relative increase in heat-absorbing black carbon aerosols has been much larger than the overall increase in the abundance of aerosols caused by human activities. Dr. Myhre discussed his work in a related podcast interview."
The clouds that highflying airplanes create, the contrails we just spoke about, also reflect sunlight away, but at night they also reflect heat back into the atmosphere. On balance, contrails are a small net contributor to warming ... if we only let planes fly during the day, they would actually help the situation, but no one is seriously thinking legislating that. Besides, as we will see, airplanes have a large negative impact on the climate because of all the greenhouse gases they produce.
It is just the sun!
So far, we have been discussing forcings that we have created: now we look at a big factor in our climate since the very first days that we had a climate: the sun itself! Included in this factor is also the earth's orbit and tilt, which also contributes to the sun's overall effect. Over the last 255 years the sun has gotten warmer. In fact over the last several billion years the sun has been getting a lot warmer. It is 35% warmer now than it was when life first appeared on earth. But in the small time frame we are looking at, and this graph is only dealing with the last 255 years, the sun's average contribution, or forcing, is quite small. Compare it to the bottom bar, which is the total of all the forcings caused by us. Since the dawn of the industrial revolution we have affected the balance by about 1.6 w/m2. The sun has only warmed up by about .1W/m2, which equals only about 7.5% of the total change we have experienced. Clearly we are having a much bigger impact on our climate than the sun.
We have just seen that it is carbon dioxide that is mainly driving the global warming we have experienced so far. Now we need to look at ... where is all this CO2 coming from? This leads us to the Carbon Cycle.
The Carbon Cycle
One of the big contributors to this store of carbon in the air is ... us! In 2007, we pumped over 8 billion tonnes of carbon up there. That was just from burning fossil fuels like oil and gas. [We are now referring to carbon, rather than CO2 but if you want to work out how much CO2 this is, just multiply the carbon amount by 3.7.] We emitted another 400 million tonnes from other sources, such as the production of cement. And that's not all: by clearing and burning forests (called Land-Use Changes below), we emitted almost 1.5 billion more tonnes. In total, just in 2007, we put up almost 10 billion tonnes of carbon into our atmosphere.
We are considered by scientists to be a major "source" of carbon.
We are not the only source of carbon. Mother Nature also puts carbon into the air. Our oceans, for example, through the decay and respiration of ocean plants and other processes, emitted over 90 billions tonnes, which makes our contribution seem tiny. However, the oceans also took out from the atmosphere over 92 billion tonnes through the creation of new life and by turning some of the carbon into carbonic acid, which is making the ocean more acidic.
The ocean is a bigger source of carbon than we are, but it is also a "sink" ... it takes carbon back out of the air; we don't. There is a myth that many people like to spread - the myth that humanity isn't creating global warming, because the oceans emit far more carbon than we do. That is true, to a point. The oceans are a greater source for carbon than we are, but it is a myth because the oceans also take back more than they emit. We don't!
Of the 10 billion tonnes of carbon that we emitted, over four and a half billion tonnes stayed in the air. This is what is increasing the levels of carbon dioxide in the atmosphere, causing our global temperatures to rise. As we have seen, the oceans took away over 2 billion tonnes. Where did the rest go?
Scientists are still debating what the mystery sink could be, but generally it is believed that the soils and land based plants have absorbed the rest of the carbon we emitted. Like the oceans, our plants and soils are sources of carbon. In 2007 the terrestrial contribution to carbon emissions was 120 billion tonnes! Decaying leaves and trees, natural fires, and burps of methane from peat bogs are just some of the causes of this vast emission of carbon. Fortunately, like the ocean, the soils and plants are also sinks for carbon, and our soils take carbon out of the air when new plants and trees grow. In 2007, the soils and plants absorbed almost 125 billion tonnes of carbon.
As we can see, while there are several big sources of carbon in the world, we are the ones who put carbon into the atmosphere, but don't take it back out.
A big concern for us all is ... when will the sinks be full? The oceans already are storing 38 trillion tonnes of carbon, in the bodies of plants and animals and as deposits of methane lying frozen on the ocean floor! Our land based plants and soils, peat bogs and permafrost hold another 6.8 trillion tonnes.
Imagine plugging up your kitchen sink, turning on the tap, and then walking away. At some point your sink will overflow. When will that happen to the oceans and soils? Already scientists have noticed that our soils are taking up less carbon than they used to, and in some places are starting to emit more carbon than they absorb. When the sinks stop being sinks, then more carbon will stay in our air, making the problem of global warming worse.
The Trends in CO2 Changes and Temperature Changes
We have been adding carbon to our atmosphere for a while now, but what was happening before we became industrialized? How does the recent rate of increase compare to the longer, historical records? This graphic illustrates pretty clearly that, before we started burning fossil fuels, and putting the carbon that had been buried in the ground over tens of millions of years into the air, the levels of CO2 in the air was pretty constant. This graphic shows the trends over the past 10,000 years. Do you notice anything significant in the last 250 years?
If we look back in time, we cannot find any time in the last 500,000 years when CO2 was higher than 300 ppm. Even if we go back 650,000 years, which scientists have done by using ice core samples in the Antarctic, the levels of CO2 have stayed in a range of 200 to 300 ppm. And yet, look at what has been happening since the industrial revolution. CO2 is reaching levels that have not been seen for millions of years. And never has the increase happened so quickly, in the matter of decades. The IPCC latest report last year said that "The recent rate of change is dramatic and unprecedented; increases in CO2 never exceeded 30 ppm in 1,000 years - yet now CO2 has risen by 30 ppm in just the last 17 years."
We have seen earlier that there are many ways for our climate to warm up...variations in the earth's orbit; increased energy from the sun can do it. But today, we are doing it by increasing greenhouse gases. As the amount of CO2 has increased, so have our temperatures
Calendar year 2008 was the coolest year since 2000, according to the Goddard Institute for Space Studies analysis of surface air temperature measurements. They reported, "In our analysis, 2008 is the ninth warmest year in the period of instrumental measurements, which extends back to 1880. The ten warmest years all occur within the 12-year period 1997-2008. The two-standard-deviation (95% confidence) uncertainty in comparing recent years is estimated as 0.05C, so we can only conclude with confidence that 2008 was somewhere within the range from 7th to 10th warmest year in the record."
This graph shows the changes over the last 130 years. This graph shows the temperature changes relative to the period from 1951 to 1980. We are today about .5C warmer than the average from 1951 to 1980. We are almost a full degree warmer than the 1880's.
The nine warmest years on record have all occurred since 1998, and the 15 warmest years have all occurred since 1990. 2007 and 2005 were the warmest years ever recorded. 2007 is even more remarkable because this record was set at a time when the solar irradiance was at a local minima and the El Nino-La Nina cycle was in its coolest phase.
Not only are we warming up, but also the pace of our warming is accelerating!
Notice the slope of the curves over time ... if we take the full 130 year period, the red line, we are definitely warming up. But now look at the blue line, which covers the last 100 years. The rate of increase is faster. Look at the orange and then the yellow lines. The yellow line shows the rate of increase over the last 25 years. Do you notice anything? Where we had a nice little slope, we now have a steep cliff. We are warming up and the rate of warming is accelerating.
There is no doubt: the climate is changing and we are warming up. And there is no doubt that we are the ones who are causing this. But, what does it mean? So what if we get a little warmer? What are the consequences of an increasingly hot environment? You are ready now to read the section on the Consequences of climate change.