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A U.S. Border Patrol agent was shot dead by assailants close to the Mexico border in southern Arizona and four suspects have been arrested, authorities said on Wednesday.

Agent Brian A. Terry was shot dead while on patrol near the border city of Nogales on Tuesday night, Tucson sector Border Patrol spokesman Mario Escalante said.

"We have four subjects in custody and we are continuing the search for more subjects," Escalante told Reuters.

Arizona straddles a furiously trafficked corridor for human and drug smugglers from Mexico.

Escalante said the FBI was leading the investigation into the shooting and would provide further details during the day.

 

Two groups of scientists are suggesting a sliver of hope for the future of polar bears in a warming world.

A study published online Wednesday rejects the often used concept of a "tipping point," or point of no return, when it comes to sea ice and the big bear that has become the symbol of climate change woes. The study optimistically suggests that if the world dramatically changed its steadily increasing emissions of greenhouse gases, a total loss of critical summer sea ice for the bears could be averted.

Another research group projects that even if global warming doesn't slow — a more likely near-future scenario — a thin, icy refuge for the bears would still remain between Greenland and Canada.

A grim future for polar bears is one of the most tangible and poignant outcomes of global warming. Four years ago, federal researchers reported that two-thirds of the world's polar bear habitat could vanish by mid-century. Other experts foresee an irreversible ice-free Arctic in the next few years as more likely.

The new study, which challenges the idea of a tipping point, says rapid ice loss could still happen, but there's a chance that the threatened bears aren't quite doomed.

"There is something that can be done to save polar bears," said lead author Steven Amstrup, the former senior polar bear scientist for the U.S. Geological Survey in Alaska. "The problem is not irreversible."

His research, published in Nature, shows there's a steady relationship between greenhouse gas emissions, sea ice and polar bear habitat. As emissions rise, sea ice and polar bear habitat decline. But unlike previous research, there's no drop-off tipping point in Amstrup's models.

Essentially until all sea ice is gone permanently in the summer there is still a chance to prevent the worst-case, if global warming is stopped in time, Amstrup's research shows.

"Such a tipping point would mean that future reductions in greenhouse gas emissions would do little to save the polar bear," said Amstrup, who is now chief scientist for the conservation group Polar Bears International. "It seems clear that if people and leaders think that there's nothing they can do, they will do nothing."

Some experts called Amstrup too optimistic, but said his computer models made sense.

"I wouldn't say that we can rule out a tipping point, but it does show that a tipping point isn't inevitable," said Walt Meier, a senior scientist at the National Snow and Ice Data Center in Boulder, Colo.

But that all hinges on reducing greenhouse gas emissions — carbon dioxide and other pollution from the burning of fossil fuels, said Mark Serreze, director of the center. "Time is running out. Humankind needs to make a choice," he said.

Time has already run out, said Henry Jacoby, a management professor at MIT and founder of its MIT Global Change Joint Program.

Jacoby examined the computer models Amstrup used in his paper and said it is based on a "world that's already long gone." The two scenarios of emission reductions are points that the world has already passed or will pass in the next few years, Jacoby said.

After the global recession led to a one-year dip in carbon dioxide emissions, they are soaring again, according to a recent study. And vague international agreements made in Cancun last week and in Copenhagen last year don't do enough, Jacoby said.

"Even given the pledges on the table, we don't come close to what these guys use in their hopeful scenario," he said.

Study co-author Eric DeWeaver of the National Science Foundation called the scenarios he used "plausible."

But DeWeaver and Amstrup agree the polar bear is in deep trouble if emissions continue to rise as they are now.

A second study was to be presented Thursday at the American Geophysical Union conference in San Francisco. That research considers a future in which global warming continues at the same pace.

And it shows that a belt from the northern archipelago of Canada to the northern tip of Greenland will likely still have ice because of various winds and currents.

The sea ice forms off Siberia in an area that's called "the ice factory" and is blown to this belt, which is like an "ice cube tray," said Robert Newton of the Lamont-Doherty Earth Observatory at Columbia University.

That "sea ice refuge" will be good for polar bears and should continue for decades to come, maybe even into the next century, he said.

Just how many polar bears could live there still has to be figured out, according to the research by Newton and Stephanie Pfirman of Barnard College.

Amstrup's study doesn't downplay the nature of global warming and its effect on polar bears, especially if emissions increase.

"The changes that are occurring in the Arctic are going on at a much more rapid rate than elsewhere in the world," Amstrup said. "So the changes that are occurring and affecting polar bears really foreshadow much more significant changes that are likely to occur worldwide."

 

The Senate cleared the decks Wednesday for a final vote on a sweeping tax package that would save millions of Americans thousands of dollars in higher taxes next year while also reducing their Social Security taxes and extending jobless benefits.

The package will then go to the House, where Democrats are fuming over extended tax breaks for the wealthy that President Barack Obama negotiated with Senate Republicans. House Democrats are considering possible changes, perhaps holding a vote to enact a higher estate tax than Obama negotiated.

A wide range of tax cuts enacted under President George W. Bush is scheduled to expire in just two weeks. The bill being voted on in the Senate would extend them for two years.

The Senate, which is expected to pass the bill with broad bipartisan support, debated several amendments Wednesday morning. One would make the tax cuts permanent, one would pay for part of the package by cutting spending elsewhere, and one would replace a proposed payroll tax cut with an extension of Obama's Making Work Pay tax credit. All the amendments were expected to fail, as the final details of the package were worked out days ago.

[Related: Sen. Thune proposes unemployment extension amendment]

Nevertheless, many House Democrats continue to oppose the estate tax provision, though leaders in that chamber say they expect the bill to ultimately pass.

"Let's find out if Republicans really want to jeopardize income tax, payroll tax and estate tax relief for every American in order to provide a budget-busting bonanza to the country's richest estates," Rep. Chris Van Hollen, D-Md., wrote in an op-ed in Wednesday's Washington Post. "House Democrats think this trade-off should be debated and voted on in the light of day."

Rep. Bill Pascrell Jr., D-N.J., said, "We can jump up and down all we want about the higher-end estate taxes, and I don't think anything's going to change because the Senate isn't going to change it."

Obama on Wednesday urged Congress to pass the plan quickly.

"I know there are different aspects of this plan to which members of Congress on both sides of the aisle object. That's the nature of compromise," the president said before meeting with business leaders. "But we worked to negotiate an agreement that's a win for middle-class families and a win for our economy, and we can't afford to let it fall victim to either delay or defeat."

House Democratic leaders say the bill could come for a vote in that chamber as early as Wednesday evening.

"I think the Senate is going to pass this measure ensuring that no one's taxes go up by a very significant margin, and I hope that our friends in the House will understand that that's the best way to go forward — simply pass the Senate bill, get it down to a president who supports the understanding," said Senate GOP leader Mitch McConnell of Kentucky.

McConnell warned House Democrats that any changes could derail the entire package.

"This agreement is not subject to being reopened," McConnell said.

The bill would extend expiring tax cuts at every income level. It also would renew a program of jobless benefits for the long-term unemployed that is due to lapse, and enact a one-year cut in Social Security taxes. The bill's cost, $858 billion, would be added to the deficit.

Some Senate Republicans are balking at the price tag, noting that Obama's deficit commission recently outlined the massive fiscal problems facing the nation. Most Senate Republicans, however, are expected to support the bill.

"The American people are going to be looking, and they're going to say, does the Senate get it? Do they understand the severity and the urgency of the problems that face our fiscal future?" Sen. Tom Coburn, R-Okla., said Wednesday.

At the insistence of Republicans, the plan includes a more generous estate tax provision: The first $10 million of a couple's estate could pass to heirs without taxation. The balance would be subject to a 35 percent tax rate.

The lower estate tax infuriated some Democrats who were already unhappy with Obama for agreeing to extend tax cuts for individuals making more than $200,000 and couples making more than $250,000.

"This administration fights for nothing," said Rep. David Wu, D-Ore.

The estate tax was repealed for 2010. But under current law, it is scheduled to return next year with a top rate of 55 percent on the portion estates above $1 million — $2 million for couples.

House Democratic leaders want to bring back the 2009 estate tax levels. That year, individuals could pass $3.5 million to their heirs, tax-free. Couples could pass $7 million, with a little tax planning, and the balance was taxed at a top rate of 45 percent.

House Democrats met in a closed-door session Tuesday evening in which member after member stood up and vented about various aspects of the bill. Other House Democrats, however, said they were eager to pass the package.

Thirty-one members of the conservative Blue Dog Democrats sent a letter to House Speaker Nancy Pelosi urging quick passage of the bill.

"It is time for us to put aside the partisan talking points and accomplish what the American people sent us here to do," said the letter.

 

The US military has dramatically stepped up air strikes and manhunts in Afghanistan in a bid to weaken the Taliban, reflecting a return to "counter-terrorism" tactics.

Dropping more bombs and carrying out more raids by special operations forces underscores a sense of urgency in the war effort, as the White House prepares to release a strategy review and commanders try to change the dynamic of a conflict mired in stalemate.

In announcing a surge of 30,000 troops a year ago, President Barack Obama embraced the idea of a "counter-insurgency" strategy that focused less on firefights with the Taliban and more on securing key towns, training Afghan forces and bolstering local government.

But the need to cut off the insurgency's supply routes to sanctuaries in neighboring Pakistan has led to a renewed emphasis on more conventional "targeting" operations, said General James Cartwright, vice chair of the Joint Chiefs of Staff.

"When we started, we probably were more aligned with counterinsurgency (strategy). The emphasis is shifting," Cartwright said last week.

"We need to reduce those lines of communication and reduce that flow to the best of our abilities," Cartwright said at an event at the National Press Club.

The balance of the US force was "starting to shift to have an element of counter-terrorism larger than we thought we were going to need when we started," he said.

The expansion of counter-terrorism raids also appears to fit in with the need to drive the Taliban to the negotiating table, as US military leaders have long stated that the insurgents must sense they are losing ground on the battlefield before they engage in genuine peace talks.

The previous commander in Afghanistan, General Stanley McChrystal, scaled back air strikes and artillery to try to reduce civilian casualties, but his successor, General David Petraeus, has taken a more aggressive approach.

US aircraft flew 850 combat sorties in November, three times the number for the same month last year, according to the US Air Force.

From January to the end of November, warplanes carried out 30,000 close air support missions for troops on the ground, a 13 percent increase compared with the whole of 2009, it said.

In the past six months, coalition forces have carried out more than 7,000 special operations missions, killing or capturing more than 600 militant leaders and inflicting heavy losses on insurgent fighters, with 2,000 rank and file soldiers killed, the NATO-led force told The Long War Journal.

More firepower will be on display soon in southern Afghanistan, where Marines will have M1A1 tanks in their arsenal -- the first use of American tanks in the war.

The intensifying pace of lethal operations has been accompanied by record casualties among US and NATO-led forces, in the most deadly year yet in the nine-year-old war with 693 soldiers killed, according to the independent website icasualties.org.

Pentagon officials say the increase in manhunts is a natural result of the troop buildup, and Defense Secretary Robert Gates said the approach in the war remained "a mix" of both nation-building and counter-terrorism.

He described the effort in eastern Afghanistan as a "disruption activity" with US troops trying to stop insurgents from coming across the Pakistani border.

In the south, US-led troops were pushing the Taliban out of towns and then holding the populated areas, he said.

"You need to understand the strategy in one part of the country will be different in another part," Gates told reporters last week on his way back to Washington.

A crucial part of the US strategy includes an expanded covert bombing campaign against Al-Qaeda and insurgent leaders in the northwest tribal belt in Pakistan.

The CIA strikes have been steadily growing, with 108 attacks by drones in 2010, compared to 53 last year, mainly in North Waziristan, where the Pakistan army has so far failed to extend an offensive against militants there, according to the New America Foundation.

The drone raids have killed 809 militants this year, compared to 405 the year before.