I don't understand what difference Keystone XL plays into climate change considering the choice is between burning American, Canadian, Norwegian, Venezuelan, or Saudi oil. The demand for fossil fuel burning will only continue to increase as global population increases, if the price increases in the short term eventually the demand will outweigh the cost from increasing population and we're back to where we were. Leaks from global increases in natural gas production is probably having a greater impact on climate change despite it being a cleaner burning fuel source - methane is quite good at absorbing infrared, far better than carbon dioxide, but we're creating carbon dioxide in much higher volumes.
Anyhow, the greenhouse effect goes back to Fourier, this isn't anything new in terms of the basic science, it's just modelling a complex system like the Earth reliably is difficult. All you can do is correlate the general trend of the system to some variables and point to them as the cause. Even if we assume the Sun is mostly responsible for global temperature rise the only variable we can have any hope in controlling is the atmospheric composition.
Otherwise if we do nothing about the issue nature might forcibly relocate us back to caves.
The solution with the least impact on our standard of living, which is also within our means to achieve is : Electric cars and electric heating sources, while investing in low or no-carbon emission sources of energy such as solar, wind, fission and fusion.
Short of massive engineering projects to reflect heat back into space, condense carbon dioxide out of the atmosphere and store it underground, or some other ridiculous proposal, the reduction of burning fossil fuels is the most practical and brings other benefits (except for oil producers).