At the nexus of the Democrats’ two most urgent policy priorities — reducing CO2 emissions and immigration reform that includes amnesty for 12 million illegal immigrants — lies an uneasy reality: Enactment of the latter may prove to be the key obstacle to achieving the former.

The economic and national security implications of open borders have been examined in depth. Less study, however, has been devoted to the possible environmental impact of immigration.

People migrate to the United States to improve their standard of living. But the liberal wish of immigration amnesty may have deleterious effects on the environment, as millions of people from developing countries settle down in, or are encouraged to move to, the world’s largest energy-consuming country and quickly embrace all the CO2-causing ways of the world’s richest economy.

This liberal conundrum is illustrated by the events going on today in the Gulf of Mexico, since a demand for fuel sparked the recent chain of events.

According to liberal wisdom, population growth is the primary cause of heavier traffic, urban sprawl, further depletion of natural resources and increased CO2 emissions. And immigration is the principal cause of U.S. population growth today. More than 1 million people become permanent U.S. residents every year, and nearly as many become American citizens.

The U.S. Census Bureau estimates that the population, more than 300 million Americans today, will grow to 400 million as early as 2030 and 420 million by 2050.

Most of this population surge is expected to be because of immigration from less developed countries. A recent Pew study projected that immigration will account for 82 percent of population growth over the next four decades.

When people move from poor countries to America, they quickly adapt in at least one way — their consumption habits. Studies show that recent immigrants’ consumption patterns, including energy use, quickly resemble those of native-born Americans.

But it is important to compare immigrants’ CO2 emissions not with those of native-born Americans but with compatriots who stay home.

In a 2008 report, “Immigration to the United States and World-Wide Greenhouse Gas Emissions,” Patrick McHugh of the Center for Immigration Studies found that, on average, immigrants increase their emissions fourfold after coming to the United States.

U.S. immigrants produce an estimated 637 million metric tons of CO2 emissions annually. That’s 482 million tons more than they would have produced had they remained in their home countries.

McHugh calculates that if the 482-million-ton increase caused by U.S. immigration constituted emissions for a separate country, it would rank 10th in the world in emissions.

Liberal policies often result in head-on collisions of some of the left’s most deeply-held values. Liberals’ embrace of doctor-assisted suicide and unlimited access to abortion disproportionately targets people with disabilities, a reality that contradicts the left’s traditional advocacy for Americans with disabilities. The left’s noisy support for free — speech on college campuses evaporates when it comes to allowing the U.S. military to recruit on campus or permitting pro-Israel groups or conservative commentators to speak freely to students.

But few other issues have made this paradox more graphic than the collision between the left’s stance on climate change and its desire to allow as many immigrants as possible into the United States.

As a conservative, I maintain a healthy skepticism of the theory of man-made global warming. I also believe that more people enjoying the fruits of modernity and economic development is a good thing — as long as those people arrived legally and obey the law.

For many on the left, however, more people living well means more people damaging the planet. That view partly explains the liberal support for abortion rights and why many on the left seem unbothered by the draconian abortion regimes of countries like China.

Environmentalists have been outspoken in their support of smaller family size and abortion rights as keys to reducing global warming. But when it comes to immigration, the single biggest contributor to population growth in the industrial world, they stand largely silent.

Some prominent liberals even suggest that the world would be better off if Americans’ standards of living resembled those of citizens of poor countries.

To take just one example, President Barack Obama’s top science adviser, John Holdren, advocated for the “de-development” of the United States in books he wrote in the 1970s.

“A massive campaign must be launched to restore a high-quality environment in North America,” Holdren wrote with Paul Ehrlich in 1973, “and to de-develop the United States … De-development means bringing our economic system (especially patterns of consumption) into line with the realities of ecology and the global resource situation.”

“It is therefore apparent,” they wrote, “that one key to saving world society lies in a measured and orderly retreat from overdevelopment in today’s [overdeveloped countries] — a process we will label, for want of a better word, de-development.”

The left offers a false choice between returning to the Dark Ages and quickly degrading the environment. A third option exists, if only liberals would stop blocking power alternatives, including nuclear power and access to numerous fuel reserves.

If “de-development” is the left’s ideal, then passing a new law that allows millions of illegal immigrants to remain in the world’s most developed country — and entices millions of others to immigrate to that country — is just about the worst thing it could do to realize it.

Gary Bauer is president of American Values and chairman of the Campaign for Working Families.