1. Climate change is happening and humans are contributing to it.

The clearest evidence that the climate is changing is the temperature record,  and globally the average temperature has risen by more than 0.7 °C over the last 100 years.

The Graph below is taken from the instrumental record of global average temperatures as compiled by NASA's Goddard Institute for Space Studies. You may notice the black line, which shows annual temperatures. Temperatures vary quite significantly over short periods of time due to a number of cyclical and other effects, such as the El Nino/ La Nina events. In order to make sense of it all, we need to take running averages over a period of at least 5 years. The red line shows running averages over 5 years.

You'll notice that the slope of the line has levelled off slightly since 1998. This is largely due to the Strong El Nino in 1998 and the predominant La Nina in recent years. La Nina events cause an upswelling of colder water from the ocean depths, causing a temporary cooling effect on average global temperatures. We are currently in a mild El Nino, so this trend is expected to change within the next few years.

Just a word about the Greenhouse effect - If it wasn't for the natural greenhouse gas effect, the Earth would be much colder than it would otherwise be. Greenhouse gases such as carbon dioxide, methane and water vapour behave like a blanket around the Earth. These gases allow the Sun's rays to reach Earth's surface but restrict the heat they create from escaping back into space.



Any increases in the levels of greenhouse gases in the atmosphere mean that more heat is trapped and global temperatures increase, which is an effect known as 'global warming'.

There is indisputable evidence from a number of independant observations that the Earth is warming. Although levels of methane have also increased, concentrations of CO2, created largely by the burning of fossil fuels, are now much higher, and increasing at a much faster rate, than at any time in the last 800,000 years. Because CO2 is a greenhouse gas, the increased concentrations have contributed to the recent warming and probably most of the warming over the last 50 years.

The actual concentration of Carbon dioxide is currently around 385 parts per million and has increased by around 30% in the last 50 years. This might seem like a very low concentration, but very low concentration can still have a significant effect. The basic physics is well established, and was first discovered by Arrhenius at the turn of the 20th Century.

References:

Stott et al 2000
http://www.sciencemag.org/cgi/content/abstract/290/5499/2133

3. The current climate change is not just part of a natural cycle

The Earth's climate is complex and influenced by many different  factors, particularly changes in its orbit, volcanic eruptions and changes in the energy emitted from the Sun. It is obvious  that the world has experienced warm or cold periods in the past without any interference from humans. The ice ages are good examples of global changes to the climate, and while it is true that regional warm periods have seen grapes grown in the UK, and mild conditions in Iceland,  the fact remains that in terms of global temperatures, the current global average temperatures are the highest recorded for at least the last 6000 years.

Over the 800,000 years covered by the ice core record, the temperature changes were primarily driven by changes in the Earth's orbit around the Sun. Over this period, changes in temperature did drive changes in carbon dioxide (CO2). Since the Industrial Revolution (over the last 100 years), CO2 concentrations have increased by 30% due to human-induced emissions from fossil fuels.

The bottom line is that temperature and CO2 concentrations are linked. In recent ice ages, natural changes in the climate, such as those due to orbit changes, led to cooling of the climate system. This caused a fall in CO2 concentrations which weakened the greenhouse effect and amplified the cooling. Now the link between temperature and CO2 is working in the opposite direction. Human-induced increases in CO2 are driving the greenhouse effect and amplifying the recent warming.

You might ask, how exactly can we measure global average temperatures before records were kept. The answer to that is by using 'proxies'. The most commonly used proxy for temperature is the deuterium proxy. This has been found to agree extremely well with measured temperatures.

Ice, like all water and ice, is made of hydrogen and oxygen. Small amounts of the hydrogen are a special heavy form called deuterium. There is a relationship between local temperature and deuterium concentration in ice collected during periods that temperature was also known. There is no reason to believe that this relationship has changed over time**, so the levels of deuterium in ancient ice can be used to reconstruct past climate. The ice also contains dust, which sometimes occurs in layers that indicate major volcanic eruptions that have spread dust around the globe. Ice cores also contain small bubbles of ancient air, which can be analyzed to discover the concentrations of gases such as carbon dioxide (CO2).

I can explain exactly how the deuterium proxy works, but I've never heard anybody question the validity of this data previously, including those who quite happily make false cause and effect claims.

Again - global CO2 and temperature  are linked. Currently the CO2 is leading the temperature rise because of the rapid rate at which its concentration is rising in the atmosphere. All natural systems have a certain lag period. The oceans have the greatest lag.

Please let me know if you would like anything explained in more detail.

** - Apart from the effects of atmospheric nuclear detonations, but these are accounted for.

5. If we continue emitting greenhouse gases this warming will continue and delaying action will make the problem more difficult to fix


The global average temperature will increase by 2 to 3 °C this century – according to one of the Intergovernmental Panel on Climate Change's (IPCC) mid-range estimates (purple line on the graph below). This rise in temperature means that the Earth will experience a greater climate change than it has for at least 10,000 years and it would be difficult for many people and ecosystems to adapt to this rapid change.

These temperature increases are likely to result in an increased frequency and severity of weather events such as heatwaves, storms and flooding. Rising levels of greenhouse gases in the atmosphere could set in motion large-scale changes in Earth's natural systems. Some of these could be irreversible - the melting of large ice sheets will result in major consequences for low-lying areas throughout the world.

Has there really been cooling since 1998?

A lot of the pseudo- sceptical articles that appear in the vast unwashed blogosphere seem to bring up this recurring myth that Global Warming has taken a break. Here are a couple of points that are important to note:

1. The discussion focuses on just a short time period starting with 1998 or later and covering at most 11 years. Even under conditions of anthropogenic global warming (which would contribute a temperature rise of about 0.2 ºC over this period) a flat period or even cooling trend over such a short time span is nothing special and has happened many times before before (for example 1987-1996). That simply is due to the fact that short-term natural variability has a similar magnitude (i.e. ~0.2 ºC) and can thus compensate for the anthropogenic effects. Of course, the warming trend keeps going up whilst natural variability just oscillates irregularly up and down, so over longer periods the warming trend wins and natural variability cancels out.

2. It is highly questionable whether this “pause” is even real. It does show up to some extent (no cooling, but a reduced 10-year warming trend) in the Hadley Center data (Hadcrut3), but it does not show in the more comprehensive GISS data. There, the past ten 10-year trends (i.e. 1990-1999, 1991-2000 and so on) have all been between 0.17 and 0.34 ºC per decade, close to or above the expected anthropogenic trend, with the most recent one (1999-2008) equal to 0.19 ºC per decade, which is just as predicted by IPCC as response to anthropogenic forcing.

The animation below shows the temperature difference between the two 5-year periods 1999-2003 and 2004-2008. The largest warming has occurred over the Arctic in the past decade, a region which is poorly represented  in the Hadley data.



Below:

Global temperature according to NASA GISS data since 1980. The red line shows annual data, the larger red square a preliminary value for 2009, based on January-August. The green line shows the 25-year linear trend (0.19 ºC per decade). The blue lines show the two most recent ten-year trends (0.18 ºC per decade for 1998-2007, 0.19 ºC per decade for 1999-2008) and illustrate that these recent decadal trends are entirely consistent with the long-term trend and IPCC predictions. Even the highly “cherry-picked” 11-year period starting with the warm 1998 and ending with the cold 2008 still shows a warming trend of 0.11 ºC per decade (which may surprise some lay people who tend to just connect the end points, rather than include all ten data points into a proper trend calculation).

In a previous post (Linked below) we saw that the Vostok Ice core records show a correlation between temperature and atmospheric CO2 as shown in the graph:
http://www.ozpolitic.com/forum/YaBB.pl?num=1247904929#3

The Vostok ice core data show a clear correlation between atmospheric CO2 concentration and global temperature. Yet phenomena that are correlated aren't necessarily causally linked. So is there a causal relationship between CO2 and temperature?

Is there a physical or chemical process by which increased atmospheric CO2 could cause increased temperature?

The answer is yes. Carbon in the atmosphere, in the form of CO2, is a greenhouse gas. A greenhouse gas is one that absorbs infrared energy. In order to understand this better we need to consider solar energy and how it interacts with the Earth. The Sun gives off a great deal of energy of different types.

Most gases, however, are transparent to IR. The three most abundant gases, Nitrogen (N2 = 78%), Oxygen (O2 = 21%) and Argon (Ar = 1%) all transmit IR energy. This is a good thing, otherwise our atmosphere would be as hot as that of Venus 477 degrees Celsius, or hot enough to melt lead.

It is also a good thing that trace gases such as CO2 and water vapour do absorb infrared energy. Without them our planet's surface temperature would be -20 degrees Celsius, which is much too cold to sustain life. The presence of trace amounts of greenhouse gases keeps the Earth habitable.

How do greenhouse gases warm the atmosphere? When IR energy of exactly the right wavelength strikes a greenhouse gas, that energy is absorbed by the bonds in the gas molecule. This causes teh molecule to vibrate and warm up. It eventually re-emits the IR energy it absorbed, allowing it to interact with other molecules.

Most of the UV, visible and IR energy from the Sun passes through the Earth's atmosphere and is absorbed by its surface (some is reflected and never absorbed). That energy heats the Earth surface to a temperature of 18 degrees C. This temperature is sufficient for the Earth to emit energy only in the IR part of the spectrum. It is this outgoing IR energy that is absorbed by the greenhouse gasses. They warm the atmosphere and re-direct the IR energy, rather than allowing it to pass uninterrupted back to space.

CO2 is considered an important greenhouse gas for two reasons 1-  it absorbs exactly the IR wavelength that the Earth emits most of, and 2- it is a gas that humans are adding to the atmosphere at an unprecedented rate.

What other observations could confirm the enhanced greenhouse effect?

The enhanced greenhouse effect should show an increase in troposheric temperatures  and a corresponding decrease in lower stratospheric temperatures. This is complicated by Ozone depletion, but we can still derive useful interpretations from satellite data.

The changes in the figure below are related to the cooling seen in the lower stratospheric MSU-4 records, but the changes there (15-20 km) are predominantly due to ozone depletion.

The higher up we go, the more important the CO2 related cooling is. It’s interesting to note that significant solar forcing would have exactly the opposite effect. In other words, it would cause a warming.