The world's two most prestigious scientific journals, the British-published Nature and the US weekly Science, asked the two contenders for their views on more than a dozen issues.

Here are extracts from their responses to Nature, which is published on Thursday. Science is published on Friday.

GLOBAL WARMING

BUSH: "Global climate change is a serious long-term issue," says Bush. He argues that scientists admit to "considerable uncertainty about the effect of natural fluctuations on climate and the future impacts climate change will have on our environment."

Bush reiterates the United States will take unilateral steps to limit the "intensity" of its greenhouse-gas emissions by 18 percent over 10 years. In 2001, Bush walked away from the UN's Kyoto Protocol pact for limiting this pollution.

KERRY: "The scientific evidence is clear that global warming is already happening and rising levels of global warming are making the problem worse." Kerry says his presidency would "take the United States back to the international negotiating table" -- but he does not specifically mention Kyoto -- and take domestic steps "to halt and reverse the growth in global warming pollution."

PREVENTING WEAPONS OF MASS DESTRUCTION

BUSH: "The key is preventing those seeking these weapons from gaining access to their most significant and technically challenging components. The redirection of former weapons scientists to productive civilian employment is a key priority."

KERRY: "It is not a problem we will be able to solve alone. It is going to require American leadership that forges an international consensus on how to deal with these weapons and the often dual-use technology that underpins them."

NEW NUCLEAR WEAPONS AND THE FUTURE OF THE THREE US NUCLEAR ARMS LABS

BUSH: Favours research into new designs of nuclear weapons. The US nuclear arsenal "[has] atrophied since the end of the Cold War and... the evolving security environment requires a flexible and responsive weapons-complex infrastructure."

KERRY: "I would end the pursuit of a new generation of nuclear weapons. Our national laboratories play a critical role in maintaining our existing stockpiles and assuring that our existing nuclear weapons are safe, secure and reliable."

US ANTI-MISSILE DEFENCE

BUSH: "Our policy is to develop and deploy, at the earliest possible date, ballistic missile defences drawing on the best technologies available."

KERRY: "I am not for rapid deployment of missile defence. We should not waste money on deployment at this point. I favour additional research, development and testing."

STEMCELL RESEARCH

BUSH: "I am committed to pursuing stem-cell research without crossing a fundamental moral line," a reference to his ban on federal funds for creating new lines of embryonic stemcells.

KERRY: "I will lift the ideologically-driven restrictions on stemcell research created by the Bush administration by overturning the ban... all [the] while ensuring vigorous ethical oversight."

SPACE EXPLORATION

BUSH: "America will return to the Moon as early as 2015 and no later than 2020, and use it as a foundation for human missions beyond the Moon. We will begin with robotic missions, and manned missions will follow."

KERRY: "There is little to be gained from a space initiative that throws out lofty goals, but fails to support those goals with realistic funding." NASA's role in exploration is vital, according to Kerry, who does not however spell out his objectives for the US space effort.

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