What causes climate change?

There are many interconnected factors that affect the Earth's climate - both natural and human induced.  But there is now strong evidence that natural causes, on their own, cannot account for the changes in climate we are seeing. A survey published in 2013 looked at over 12,000 peer-reviewed climate science papers and found that there was an almost unanimous agreement (97% consensus) that humans are the dominant cause of global warming.

Natural causes
Some changes to the Earth's climate are caused by the natural interactions between the sun, land, oceans and the atmosphere. These occur over very long periods of time and include:

• Changes in strength of the Sun   
  All of Earth’s energy comes from the Sun.  But, energy output of the Sun is not constant and varies over time.
   
• Changes in Earth’s orbit around the Sun.
  Over about 100,000 years, the shape of Earth’s orbit changes from being less elliptical to more elliptical and back. This causes the distance between the Earth and Sun to fluctuate.  When the Earth is closer to the Sun, its climate is warmer.

  The axis around which the Earth rotates also undergoes cyclic change (22.1° to 24.5°) over about 41,000 years. When the angle increases, the summers become warmer and the winters become colder. These cycles (often referred to as Milankovitch Cycles) effect the amount of solar energy reaching the Earth.
   

• Movement of tectonic plates
  Movement of tectonic plates can cause continents to shift into different climates, as well as form volcanoes and mountains. Although this is a very slow process, it can contribute to a change in the climate over time.  Large mountain chains can disturb the circulation of air and volcanoes eject greenhouse gases and dust particles into the atmosphere during eruptions.
   
• Ocean currents
  Oceans store a large amount of heat, so even small changes in ocean currents can have a significant impact on the global climate.

Human impact

There are natural sources of carbon dioxide (e.g., respiration (breathing out), volcanic degassing) and natural sinks of carbon dioxide (e.g. photosynthesis, rock weathering).  Over the long term, they have tended to balance which has created a remarkable stable climate over the Earth’s history, working like a thermostat.

However, human activity has changed the concentration of greenhouse gases in the atmosphere in two important ways:

• We have cut down forests to develop land for agriculture. Trees absorb carbon dioxide so, with fewer trees, more carbon dioxide builds up in the atmosphere. Also, the agriculture that replaces the forests can often be a source of emissions.
   
• By burning fossil fuels like coal, oil and gas to meet the increasing demand for energy, we release greenhouse gases. We have been burning historic stores of carbon much faster than they were created.

Naturally, less than 1 gigaton per year of carbon is removed from the atmosphere into rocks.  In comparison, we are emitting 9 gigaton per year of carbon. We are adding to atmospheric carbon reservoir, enhancing the greenhouse gas, at an alarming rate. Far quicker than the ‘natural thermostat’ can handle.

Unfortunately, this extra CO2 we have created stays in the atmosphere for thousands of years. About 1/3 of the extra CO2 we released into the atmosphere goes into the ocean, limiting the enhanced warming but making the oceans more acidic and impacting the ecology. Oceans are likely to eventually reach a saturation point where they can absorb no more.

Feedback loop and tipping point.
Initial warming can be amplified by certain processes:

• Melting sea ice exposes darker ocean water reducing the amount of heat reflected into space.
• Melting Arctic permafrost releasing CO2 and methane (another greenhouse gas) 
• Warming and acidification of the oceans reducing its capacity to absorb CO2. Therefore, more CO2  in the atmosphere to trap heat.  

These processes are called positive feedback loops. At some point, these feedback loops could overpower our efforts to cut down our emissions – the ‘tipping point’.  In this case, even if we get to zero carbon emissions, we will have set off unstoppable, runaway climate change.