Climate Change is the most important threat to our future. As all environmental problems, climate change, has many causes which many times function as cause and effect to other environmental problems. Global Warming is the direct cause of climate change but, as said before, it is a consequence of other problems and antropogenic activities. In one hand, the emission of GHG, such as CO2; and, on the other, not enough capture of it. The International Energy Agency states that most of this kind of emissions come from transportation. Therefore, I would consider transportation (which generally requieres fossil fuel combustion) as the one of the most important activities which contribute to climate change. Also, deforestation is an important cause (since there isn't enough capture of CO2). Therefore, it is important that national and international strategies on Climate Change should also try to attack the deforestation problem, and give more importance to the United Nations REDD Program.

In the case of the UK (my home) there is the UK National Atmospheric Emissions Inventory which presents a pretty complete breakdown of greenhouse gas emissions.

This is particularly useful, because they describe the methodology used to gather the stats (meaning you can have more confidence in them), and provide almost raw numbers in their reports - which are detailed to an almost mind numbing level.

One question I guess, is what percentage of the overall climate change is down to simple emissions - it's certainly a large part, however, we are doing more damage through something like deforestation, which has massive knock on effects beyond the immediate emission of GHGs.

Personally, I think stopping the direct burning of fossil fuels for large scale energy is the single biggest win we could easily have - and in this, China, the USA and Australia have to act.

Global Warming is a biggest cause for the climatic change.

Visit the link http://www.copenhagenclimatechallenge.org/index.php?option=com_content&view=article&id=62:causes-of-climate-change-not-settled-scientists&catid=1:latest-news where you have the evidence for the factors affecting climatic change.

Human emissions of carbon dioxide and other ‘greenhouse gases’ (GHG) are dangerously impacting climate

Computer-based models are reliable indicators of future climate.

Crudely speaking the biggest shares of emissions are divided between:

Agriculture, Transport, Industry, Power and Domestic (particularly space heating and hot water in homes)

The current conventional wisdom observes that decarbonising the power sector enables other sectors (such as switching to electric heating in homes, or to electric vehicles in transport) to decarbonise. If anyone has any comments about this, I'd like to hear them.

Worldwide, the main sources of greenhouse gases are: fuels combustion in electricity generation and fuels combustion in vehicles use.

Other very important sources are oil and gas production, livestock, deforestation and forest degradation, agricultural production of some grains, use of fertilizers, solid waste (mis)management, and water treatment.

But the relative importance and contribution of each of these activities varies by country, according to the type of infrastructure is has for electricity generation, or waste management. The industrialised countries prepare and publish emissions inventories every year. You may find them here: http://unfccc.int/national_reports/items/1408.php Look for "GHG Inventories".

Not all countries prepare inventories in an annual basis. Some have prepared one or two, just to have a rough idea of how much they emit and from which sources.

Given that combustion of fossil fuels is the main source of greenhouse gas emissions, the International Energy Agency (IEA) prepares and publishes its own estimate of emissions based on data on fuels production and trade provided by the country members to IEA. Look here for their statistics: http://www.iea.org/stats/index.asp

Also the Energy Information Administration (EIA) from the United Sates, prepares and publishes similar estimates, based on data collected by them. Look here: http://www.eia.doe.gov/environment.html

Other independent estimates are done by the World Resources Institute, a non-governmental organisation that works on different topics, including climate change: http://earthtrends.wri.org/searchable_db/index.php?theme=3

The other side of the coin is how the CO2 is captured and retained by natural means... that is part of the CO2 natural cycle. Here, the important part is that the oceans, the forested areas, and the soils play an important part. If they are degraded somehow, their natural ability to capture and retain CO2 diminishes. Some estimates on this regard are done by the International Geosphere-Biosphere Programme (IGBP) through the Global Carbon Project (GCP). They publish the "Carbon Budget", an estimate on how the natural capacity to deal with increasing emissions is changing. Look here: http://www.globalcarbonproject.org/carbonbudget/08/presentation.htm

Hope this helps

I have read and heard about the impact of raising livestock being the most significant in terms of GHG emissions that contribute to climate change.

This article tries to diminish the impact, quoting a 3% contribution: http://futurity.org/earth-environment/dont-blame-cows-for-climate-change/

However, that 3% is typically from the methane that is directly produced by cattle. When considering the total impact of breeding, feeding / grazing, and transporting animals in addition to slaughtering, packing, and delivering their meat, the numbers skyrocket. I have heard estimate as high as 20% of total GHG output in the world!

This article from 2006 discusses the impact of deforestation to create land for grazing and crop growth: http://www.fao.org/ag/magazine/0612sp1.htm. They cite an 18% overall GHG contribution if the entire commodity chain is considered. Here's a quote: "Grazing occupies 26 percent of the Earth's terrestrial surface, while feed crop production requires about a third of all arable land."