Undeniable signs of climate change: the human fingerprints

We are now at the stage in history where only the most ardent flat-earther or conspiracy theorist can deny human influence over the climate in recent centuries.

There are many individual strands of evidence that are compelling in themselves, but when put together make up a completely damning picture which points only to one thing: man made climate change is here, it’s real, and it’s happening faster than was thought possible only a few years ago.

The driver, of course,  is the consistent rise in greenhouse gases, predominantly CO₂, consistent with what one would expect from with huge levels of fossil fuel combustion. Levels have been climbing rapidly (see graph below) and are now at levels last seen 15 million years ago.

Source: http://en.wikipedia.org/wiki/File:Co2-temperature-plot.svg

 Though it doesn’t happen immediately, atmospheric temperatures increase after CO₂ is emitted, as has happened in the past. The graph below shows  how temperatures have risen in the last half-century.

But rising temperatures are only one way that we are influencing the planet. In 2010 the National Oceanic and Atmospheric Administration (NOAA) released a report highlighting the state of the warming climate. Over 300 scientists from 48 countries contributed to the report, which highlighted ten ways in which the environment is changing due to rising temperatures, including receding glacier cover and sea ice levels, and increasing air temperature, sea temperature, and humidity.

Even the deniers can no longer deny climate change. The Berkeley Earth Project was set up to reassess evidence of the world's warming. It was supported and funded by climate change sceptics (including the Koch Brothers). In October 2011 they released their findings that, instead of contradicting existing measurements, correlated with them extremely closely, supporting the climate science consensus. http://www.bbc.co.uk/news/science-environment-15373071 Whether this puts an end to the sceptics once and for all remains to be seen.

Quick question and answer: how do we know humans are causing climate change?

Could the rise in atmospheric carbon be coming from somewhere else?

• Humans are currently emitting around 30 billion tonnes of CO2 into the atmosphere every year. Of course, it could be coincidence that CO2 levels are rising so sharply at the same time so let's look at more evidence that we're responsible for the rise in CO2 levels.
• When we measure the type of carbon accumulating in the atmosphere, we observe more of the type of carbon that comes from fossil fuels
• This is corroborated by measurements of oxygen in the atmosphere. Oxygen levels are falling in line with the amount of carbon dioxide rising, just as you'd expect from fossil fuel burning which takes oxygen out of the air to create carbon dioxide
• Further independent evidence that humans are raising CO2 levels comes from measurements of carbon found in coral records going back several centuries. These find a recent sharp rise in the type of carbon that comes from fossil fuels

How do we know that the extra CO2 in the atmosphere is warming the planet through the greenhouse effect?

• CO2 absorbs heat at particular wavelengths. Satellites measure less heat escaping out to space, at the particular wavelengths that CO2 absorbs heat, while surface measurements show more heat returning at CO2 wavelengths.
• If an increased greenhouse effect is causing global warming, we should see certain patterns in the warming. For example, the planet should warm faster at night than during the day. This is indeed being observed.
• Another expected result of greenhouse warming is cooling in the upper atmosphere, otherwise known as the stratosphere. This is exactly what's happening .
• With the lower atmosphere (the troposphere) warming and the upper atmosphere (the stratosphere) cooling, another consequence is the boundary between the troposphere and stratosphere, otherwise known as the tropopause, should rise as a consequence of greenhouse warming. This has also been observed.
• An even higher layer of the atmosphere, the ionosphere, is expected to cool and contract in response to greenhouse warming. This has been observed by satellites. 

(The above Q&A was taken from Skeptical Science, where you can read more about the evidence)

Sources and further reading
Last Time Carbon Dioxide Levels Were This High: 15 Million Years Ago, Scientists Report
http://www.sciencedaily.com/releases/2009/10/091008152242.htm

Climate change: Key Indicators
http://climate.nasa.gov/keyIndicators/

Does CO2 always correlate with temperature (and if not, why not?)
http://www.skepticalscience.com/co2-temperature-correlation.htm

NOAA: Past Decade Warmest on Record According to Scientists in 48 Countries
http://www.noaanews.noaa.gov/stories2010/20100728_stateoftheclimate.html

BBC News - Global warming 'confirmed' by independent study
http://www.bbc.co.uk/news/science-environment-15373071