The United States and China are the world's two largest economies, two largest consumers of energy, and two largest emitters of greenhouse gases. Together we account for about 40 per cent of the world's emissions.
We need to solve this problem together because neither one of us can solve it alone. Even if the US somehow eliminated all its domestic greenhouse gas emissions, it still wouldn't be enough to counteract the carbon pollution coming from China and the rest of the world.
Likewise, even if China went down to zero emissions, it wouldn't make enough of a difference if the US and the rest of the world didn't change direction.
That's the reality of what we're up against. That's why it matters that the world's most consequential relationship has just produced something of great consequence in the fight against climate change.
On Wednesday, US President Barack Obama and Chinese President Xi Jinping jointly announced targets to reduce carbon emissions in the post-2020 period. By doing this - together, and well before the deadline established by the international community - we are encouraging other countries to put forward their own ambitious emissions reduction targets soon and to overcome traditional divisions so we can conclude a strong global climate agreement in 2015.
Our announcement can inject momentum into the global climate negotiations, which resume in less than three weeks in Lima, Peru, and culminate next year in Paris. The commitment of both presidents to take ambitious action in our own countries, and work closely to remove obstacles on the road to Paris, sends an important signal that we must get this agreement done, that we can get it done, and that we will get it done.
This is also a milestone in the US-China relationship, the outcome of a concerted effort that began last year in Beijing, when State Councillor Yang Jiechi and I started the US-China Climate Change Working Group. It was an effort inspired not just by our shared concern about the impact of climate change but also by our belief that the world's largest economies, energy consumers and carbon emitters have a responsibility to lead.
The targets themselves are also important. Ambitious action by our countries together is the foundation to build the low-carbon global economy needed to combat climate change.
The US intends to reduce net greenhouse gas emissions by 26 to 28 per cent below 2005 levels by 2025 - a target that is both ambitious and feasible.
It roughly doubles the pace of carbon reductions in the period from 2020 to 2025 as compared to the period from 2005 to 2020. It puts us on a path to transform our economy, with emissions reductions on the order of 80 per cent by 2050.
It is grounded in an extensive analysis of the potential to reduce emissions in all sectors of our economy, with significant added benefits for health, clean air and energy security. Our target builds on the ambitious goal Mr Obama set in 2009 to cut emissions in the range of 17 per cent below 2005 levels by 2020.
The US is on track to meeting that goal, while creating jobs and growing the economy, with the help of a burgeoning clean energy sector. Since Mr Obama took office, wind energy production has tripled and solar energy has increased by a factor of 10. This summer, the Environmental Protection Agency proposed the first carbon pollution standards for existing power plants, which account for a third of US carbon pollution.
The Chinese targets also represent a major advance. For the first time China is announcing a peak year for its carbon emissions - around 2030 - along with a commitment to try to reach the peak earlier. That matters because over the past 15 years, China has accounted for roughly 60 per cent of the growth in carbon dioxide emissions worldwide.
We are confident that China can and will reach peak emissions before 2030, in light of Mr Xi's commitments to restructure the economy, dramatically reduce air pollution and stimulate an energy revolution. China has also announced that it will expand the share of total energy consumption coming from zero-emission sources (renewable and nuclear energy) to around 20 per cent by 2030, sending a powerful signal to investors and energy markets around the world and helping accelerate the global transition to clean-energy economies.
To meet its goal, China will need to deploy an additional 800 to 1,000 gigawatts of nuclear, wind, solar and other renewable generation capacity by 2030 - an enormous amount, about the same as all the coal-fired power plants in the country today, and nearly as much as the total electricity generation capacity of the US.
There is no question that all of us will need to do more to push towards the de-carbonisation of the global economy. But in climate diplomacy, as in life, you have to start at the beginning, and this breakthrough marks a fresh beginning.
Two countries regarded for 20 years as the leaders of opposing camps in climate negotiations have come together to find common ground, determined to make lasting progress on an unprecedented global challenge. Let's ensure that this is the first step towards a world that is more prosperous and more secure.
NEW YORK TIMES
The writer is the United States Secretary of State.