President Barack Obama should flat-out reject TransCanada's proposed Keystone XL oil pipeline.

The proposed 1,700-mile pipeline would link Canada's Alberta tar sands to refineries and ports on the Gulf of Mexico. Obama has less than 60 days to decide whether to approve the project, thanks to Congressional Republicans who tacked the 60-day ultimatum onto the payroll tax-cut extension passed last month.

Obama isn't expected to consider authorizing Keystone XL until environmental reviews are completed. But he should just kill it. There are plenty of good reasons to do so, most of them related to the potential environmental damage.

For starters, the pipeline's current proposed route crosses through Nebraska's vast Ogallala Aquifer. Many Nebraskans, including some elected Republicans, have rightly expressed concern about the prospect of spills within the aquifer, a major source of drinking and irrigation water.

Environmentalists also worry about the effects that extracting oil from tar sands could have on the areas surrounding the mining sites, including water pollution and deforestation.

But the pipeline proposal is, at its core, an international climate issue. Keystone XL would increase export opportunities for tar-sands crude by providing a direct route from the sands to Texas refineries and ports. Compared to conventional oil production and consumption, tar sands are dirtier and generate substantially more greenhouse gas emissions. Some studies estimate that a barrel of tar-sands crude generates several times the greenhouse gas emissions of a barrel of conventional oil.

The tar sands - which technically do not contain tar - yield a petroleum source called bitumen, which is essentially a molasses-like heavy crude. The bitumen is extracted in one of two ways.

One method is to dig up the sand, then use hot water to separate the bitumen from it.

But for deeper bitumen, high-pressure steam is forced into the deposit, thinning the bitumen so the sludge can be sucked up to the surface. That's an energy-intensive process that consumes natural gas.

In fact, a 2009 Greenpeace report says natural gas is used in almost every step of producing oil from tar sands. For example, the bitumen is too thick to pipe to refineries, so facilities called upgraders have to thin it. That process uses natural gas.

The same report says US refineries consume higher amounts of energy when they process the tar-sands crude.

The tar sands aren't the only unconventional fossil-fuel source that energy companies are interested in. Stateside, it's been shale gas, as in Marcellus Shale. A Cornell University professor's study showed that shale gas, from production to consumption, generates more greenhouse gas emissions than conventional natural gas. There's also serious concern that the favored extraction method, hydraulic fracturing, can lead to contaminated water supplies.

See a pattern here?

Companies and countries have already tapped the easy reserves. In the process, populations across the world got hooked. Consumption keeps increasing, whether through fuels or manufactured products like fertilizer and plastics.

The US Energy Information Administration predicts that in 2035, global energy consumption will have doubled since 1990. The EIA also predicts that global greenhouse gas emissions will increase substantially; that's bad news for the climate.

Global dependence on fossil fuels, however, means money for energy companies. That's why they're looking at tar-sands oil and shale gas. Even a decade ago, these sources were not economically practical for companies to exploit.

Now that prices are higher and demand remains strong, however, the companies can turn a tidy profit from these products. That happens at the expense of the environment.

There's a strong case to be made that the pipeline, and oil from the tar sands, isn't in the best interests of the US and also the world. A better solution would be to invest in and advance energy conservation and renewable energy sources.

Let's hope Obama sees it that way too.