Al Gore, former vice-president

This was a terrific and historic speech, by far the best address on climate by any president ever.

I applaud the new measures announced by President Barack Obama this afternoon to help solve the climate crisis – particularly the decision to limit global warming pollution from existing as well as new power plants.

Following the important pledges he made in both his inaugural address and State of the Union speech earlier this year, and the historic gains in renewable energy and fuel efficiency that the President delivered in his first term, the policy changes he announced today represent important steps forward in the battle to halt catastrophic climate disruption.

Most importantly, President Obama has directed the Environmental Protection Agency to establish regulations on the amount of global warming pollution existing fossil fuel plants can pour into our atmosphere.

This action – if followed by skillful and thorough execution of the plan – has the potential to fundamentally alter the course of our nation's energy infrastructure development and help to promote a sustainable future. On the international front, this action will bolster U.S. credibility and moral authority in negotiations with other countries.

Nicholas Stern, chair of the Grantham Research Institute on Climate Change and the Environment at London School of Economics and Political Science

President Obama's speech was very clear on the scale of dangers posed by climate change and the responsibility of the United States and other countries to act. Recent experience shows that the United States can cut emissions and grow. The United States, with its technology and entrepreneurship, can lead this new low-carbon growth story. The president was right to place great emphasis on standards for cleaner power plants, accelerating renewables, energy efficiency standards in buildings, vehicles and appliances, and the importance of leadership by the public sector in implementing these standards. The president recognised as well the role of natural gas, but also saw it in a medium-term role and as bridge to cleaner technologies.

There are will be new jobs in the United States in cleaner and more efficient technologies, and investment opportunities round the world. The low-carbon economy can create enormous potential growth for the private sector in the United States, and around the world.

I am glad that President Obama emphasised the need for free trade in clean goods and services, and acknowledged the benefits of collaborating with China on phasing out HFCs. China and the United States can and should both go further in tackling climate change, for instance by increasing energy efficiency, and phasing out coal. China has been moving very strongly on this issue both in its current five-year plan and in preparations for the 13th five-year plan. There would be tremendous benefits if China and the United States could together show real international leadership on this issue. And it is time for Europe to stop hesitating and once again position itself at the forefront of global efforts.

I welcome the president's emphasis on help for developing countries to avoid the mistakes made by the rich industrial countries, including by sharing technologies and trying to find alternatives to coal. He is right to place an emphasis on finding ways to work together internationally and to reach an agreement between all countries at the United Nations climate change summit in Paris in 2015.

I am glad that the president chose to close his speech with an appeal to young people to make the case and to press politicians for action. Our generation will be judged by the inheritance we leave.

Jessy Tolkan, board member, 350.org and former executive director of Energy Action Coalition.

I've waited four years and 158 days to hear the president give this kind of speech on climate change. This is the kind of leadership my generation expected when we first elected, and then re-elected President Obama to office. This speech has been a long- and hard-time coming, but the president's plan outlines meaningful action that begins to meet our global and moral responsibility to address climate change. It's refreshing to see this second-term president finally step into his administrative powers and take the reigns in this fight.

Today's announcement should be credited to the tens of thousands of Americans who have relentlessly applied pressure to this administration to keep this critical issue alive. It's an indication that our voices can penetrate through the powerful of influence of dirty energy. The fight is long from over. The president threaded a needle in proclaiming that the Keystone pipeline should not be approved if it increases carbon pollution. We know the pipeline produces undoable harm to our environment. No decision, other than denying Keystone will be an acceptable follow-up to this speech.

As the president spoke today, 500+ young climate leaders from more than 100 countries are convened in Instanbul, Turkey, for Global Power Shift. This generation is counting on this speech marking the beginning of a new era in the fight to stop climate change.

Van Jones, co-founder Rebuild the Dream and former White House green jobs advisor

I think there was an important development on the Keystone pipeline, and I think it's worth spending some time reading the tea leaves on that. I think the president gave a very encouraging wink and a nod to environmentalists on this issue. I think that if I were the head of TransCanada, I would have my head in my hands, and that if I were (climate activist) Bill McKibben I would have my fist in the air after that speech.

There is no conceivable way you can calculate this without a tripling of tar sands production, which can not be climate neutral. I think that if you read the tea leaves here, the president has kicked this argument up and away from the jobs argument and put it down on the carbon side of the discussion, and that is probably ultimately favourable to environmentalists.

I think the tables are turning against Keystone. I don't think the president wants to lose his youth base by being a president who is spying on them while cooking the planet with a dumb pipeline that everybody hates. I just don't think he wants to do that.

Christiana Figueres, executive director of the UN's climate secretariat

President Obama's climate action plan is a necessary next step to meet an immediate, worrying shortfall in action to deal with climate change and can be a critical move forward on the path towards a new, global climate agreement. It remains vital that the United States as the world's largest developed economy is seen to be leading serious action to deal with climate change, both at home and abroad. These new steps will help to meet those goals, if they are implemented to the fullest extent to which they are intended.

It is significant that the new plan aims to start up rapidly and covers the full menu of solutions to climate change: clean energy, renewable energy, energy efficiency and the many actions that all countries need to take to adapt to accelerating climate change. This climate action plan should be positive for the US economy and the economies of other countries, as the US shifts faster towards a sustainable, low carbon model, including addressing directly the heaviest sources of emissions from unmodified coal and gas plants.

Bob Ward, policy and communications director at the Grantham Research Institute on Climate Change and the Environment at London School of Economics and Political Science

President Obama's excellent speech laid out very clearly the choice that the American people face. A high-carbon economy and unmanaged global warming will create increasing risks from rising sea levels and changes in extreme weather, fundamentally undermining the prospects for future prosperity. But investing in a transition to a low-carbon economy offers a more secure and sustainable source of growth.

The question now is whether moderate Republicans will have the courage to break from the extreme political ideology that has straitjacketed them in denial of the scientific evidence. President Obama has laid out his personal plan for tackling climate change, but without the support from Congress for federal action, the United States will miss the opportunities that will be offered by the low-carbon industrial revolution.

He must continue to take his argument to the American people and to force all lawmakers to face up to the task of reducing greenhouse gas emissions and adapting to those impacts of climate change that cannot now be avoided. President Obama clearly wants the United States to offer international leadership on climate change. It is time for Congress to share his ambition.