what is thought to cause global warming?

The planet Venus is indeed a frightening example of what scientists call "runaway global warming." Venus actually had oceans of water at one time, but now is an intense 900ºF, which has boiled away all surface water. Astronomers attribute the extreme hotness of Venus's atmospheric temperature to the occurrence of a runaway greenhouse effect on Venus. Ninety-six percent of Venus's atmosphere is now carbon dioxide - a powerful greenhouse gas. On Venus (and on Earth), sunlight passes through the atmosphere as visible light, which warms up the surface of the planet. On Earth, some of this surface heat is then radiated back into space, mostly in infrared wavelengths. But unlike visible light waves, infrared waves can't pass through Venus's atmosphere back into space because they are blocked by the huge amount of carbon dioxide and other greenhouse gases in its atmosphere. As a result, the heat is "trapped" in Venus atmosphere, causing it to be the hottest place in the solar system. And Venus keeps getting hotter as more sunlight hits it every day. Even' worse, scientists say the "runaway greenhouse effect" on Venus could continue indefinitely.
If all of the carbon dioxide, methane, and other greenhouse gases currently trapped in Earth's oceans and other greenhouse gas "sinks" were released into the atmosphere through the positive feedback mechanisms discussed in the articles below, our atmosphere would have as much carbon dioxide and other greenhouse gases as Venus now does.
The vast majority of the world's climate scientists agree that global warming is now occurring on Earth due to the release of carbon dioxide and other greenhouse gases into the atmosphere by the burning of fossil fuels and other human activities. It is not certain that the present global warming will "runaway" and cause a catastrophic climate change such as that which has made Venus uninhabitable, but, as is discussed in the articles below, it is considered a possibility. The question is, do we want to play what is basically a form of Russian roulette with life on Earth by continuing to pour greenhouse gases into the atmosphere on the gamble that runaway global warming won't happen? (06/01/1992) Scientists fear a coalescing pattern of positive global warming feedbacks could cause "runaway" global warming, threatening the entire basis for life on Earth. Scientific consensus on global warming was formalized in May 1990 in an assessment by the Intergovernmental Panel on Climate Change (IPCC). Its authoritative early warning set most of the world's governments negotiating for a Global Climate Convention in February 1991. Amid rapid advances in scientific research, the IPCC was asked to prepare an updated report by January 1992.

The climate system varies both through natural, "internal" processes as well as in response to variations in external "forcing" from both human and non-human causes, including solar activity, volcanic emissions, variations in the earth's orbit ("orbital forcing") and greenhouse gases. Climatologists agree that the earth has warmed recently. The detailed causes of this change remain an active field of research, but the scientific consensus identifies greenhouse gases as the primary cause of the recent warming. Outside of the scientific community, however, this conclusion can be controversial.
Adding carbon dioxide (CO2) or methane (CH4) to Earth's atmosphere, with no other changes, will make the planet's surface warmer; greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be an estimated 30 °C (54 °F) lower, and the Earth uninhabitable. It is therefore not correct to say that there is a debate between those who "believe in" and "oppose" the theory that adding carbon dioxide or methane to the Earth's atmosphere will, absent any mitigating actions or effects, result in warmer surface temperatures on Earth. Rather, the debate is about what the net effect of the addition of carbon dioxide and methane will be, when allowing for compounding or mitigating factors.
One example of an important feedback process is ice-albedo feedback. The increased CO2 in the atmosphere warms the Earth's surface and leads to melting of ice near the poles. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and the cycle continues.
Due to the thermal inertia of the earth's oceans and slow responses of other indirect effects, the Earth's current climate is not in equilibrium with the forcing imposed by increased greenhouse gases. Climate commitment studies indicate that, even if greenhouse gases were stabilized at present day levels, a further warming of perhaps 0.5 °C to 1.0 °C (0.9–1.8 °F) would still occur.