What a difference a week makes. This morning we awoke to the news that President Obama and Secretary of State Kerry have negotiated a historic joint announcement on climate change and clean energy cooperation. Coming from the world's two largest economies and two biggest carbon emitters, the new targets set by President Obama and President Xi Jinping have put the international community on notice: It's time to put up or shut up.

Three major, overarching goals were announced:  

• The U.S. will cut its net greenhouse gas emissions to between 26 and 28 percent below 2005 levels by 2025.
  
  
• China will attempt to peak its CO2 emissions by 2030 (and possibly sooner).
  
  
• Also by 2030, China will increase the share of non-fossil fuel energy it uses to around 20 percent.

China's pledge to cap its emissions is momentous -- and should compel India and other developing nations to set their own ambitious targets. But the game changer in this announcement -- and an underreported one at that -- is China's goal of producing 20 percent of its electricity from carbon-free sources by the end of the next decade. To accomplish that, China will need to install 800 to 1,000 gigawatts of energy with zero emissions by 2030 -- an amount almost equal to current total U.S. electricity generating capacity.

Such rapid clean-energy growth will accelerate a positive feedback loop. As China drives toward its goal, clean energy prices will continue to drop. Solar and wind are cheaper than fossil fuels in many places already; as prices plummet even further, the transition from dirty fuels will pick up speed, helping China, the U.S., and other countries meet and exceed their climate targets and save money in the process.

The U.S. and China aren't acting out of sheer altruism, though -- both countries will also gain tremendously by leading the transition to a clean-energy economy. Sure, cutting carbon pollution is a driving factor, but there's enormous benefit in doing so. Fighting climate change is something that we get to do, not just something that we have to do. According to the Center for American Progress, an accelerated transition to clean energy in this country will create 2.7 million new jobs in the clean energy sector nationwide. No doubt the Chinese have performed a similar calculus.

Of course, China had already made it clear that renewable energy was a national priority. At a time when we face yet another congressional debate over whether to renew the Production Tax Credit for wind power in this country, China is erecting wind turbines like yard signs in a swing state -- it already has more wind power than the entire European Union, and will install a record amount of both solar and wind again this year.  

So, yes, while this agreement means that China and the U.S. are standing together to take responsibility for climate action, this partnership is just as much about opportunity. That, more than anything, is why clean energy is unstoppable. The opportunities it represents -- both economically ("Consumers and businesses will save literally billions of dollars," said one administration official) and in so many other ways -- are a powerful force for bringing people, industries, and, in this case, nations together. Just this week, for instance, I attended an event highlighting how the International Brotherhood of Electrical Workers and the Sierra Club have worked together to help create more than a thousand new construction jobs with good wages and benefits through responsibly sited large-scale solar projects in California.

One more point on this announcement. Those who keep a clear, unflinching eye on the total carbon reductions needed to keep warming below 3.6 degrees Fahrenheit will say that the U.S. could in fact do much more than cut its carbon by 26-28 percent a decade from now. They're right. The EU has indicated it will cut its carbon pollution by 40 percent (by 2030) -- using a more challenging baseline figure. And our fragile planet certainly needs China to cap its emissions sooner than the end of the next decade.  

But even a journey of a thousand miles begins with a single (in this case, giant) step. Two centuries ago, Napoleon presciently compared China to a sleeping giant that would one day awaken and shake the world. But he also made an observation about leadership: "One can lead a nation only by helping it see a brighter future -- a leader must offer hope." President Obama and President Xi Jinping offered that hope today by stepping forward together.

And what about the Republican leaders gnashing their teeth in Congress? What message of opportunity are they offering? How, exactly, do they propose to lead the nation forward when their rallying cry is "Retreat!"?

Don't ask me. I'm not a scientist. But I do know that real leadership should be acknowledged when it happens. For all those who have marched, testified, lobbied, litigated, invested, divested, tweeted, posted, and donated to fight against dirty fuels and for 100% clean energy, take a bow. We're building momentum. And also, please take a moment and send President Obama a message thanking him for acting on climate and elevating our clean energy ambitions.

My current column in Sierra magazine ("Money Talks, Carbon Walks"), describes how each of us can help build the fossil-free economy by exercising our influence as consumers and investors. Most of us will do that because we believe it's right but, as I wrote in Sierra: "If environmental concerns aren't reason enough to divest from the dirty energy sector, do it out of selfishness, because companies that depend on their fossil fuel reserves for future earnings are simply a bad investment these days."
 
That gets to a bigger point. Some companies look at the future and prepare not only to adapt but also to thrive. Others can't shake their ties to the past, whether that's digging up fossil fuels or manufacturing buggy whips. Guess which ones have proven to be better long-term investments?
 
The Koch brothers won't be happy to hear it, but economic success in the future will require a commitment to sustainability -- whether you're talking about clean energy, clean water, or fair labor practices. Companies that haven't figured that out by now are already falling behind, whether they know it or not
 
We don't have to wonder which companies "get it." A new report from Ceres, a nonprofit organization that advocates for sustainability leadership, examines the sustainability record of 613 U.S. companies. Together, these companies account for almost 80 percent of the total market capitalization of all public companies in the country.
 
As the report's title ("Gaining Ground") suggests, the overall trend is positive. Companies are paying more attention to sustainability issues than they were two years ago. Seven out of ten, for instance, have at least some kind of strategy for reducing greenhouse-gas emissions. What tempers that good news, though, is that progress is not happening anywhere near fast enough. We're already feeling the effects of climate disruption. The deadline for reducing emissions fast enough to avoid a climate catastrophe is, as they say, nonnegotiable.
 
It's not too surprising that many of the companies that are furthest ahead of the pack on reducing greenhouse gas emissions and adopting renewable energy are from the high-tech sector. Some of these companies were prompted by direct advocacy by Greenpeace; others were motivated to respond to concerns of shareholders, and the values of their own employees. Each company knows well the importance of adapting to new paradigms. They also use a lot of energy. Increasingly, though, that energy is renewable.
 
Google has committed over $1 billion to renewable energy projects such as large-scale wind and rooftop solar. These projects will generate far more electricity than it uses for its own operations. Apple Computer recently announced that 94 percent of its corporate facilities and 100 percent of its data centers are powered by clean energy sources.
 
But the food and beverage sector is the one where the highest percentage of companies has set formal, time-bound emissions-reduction targets. The reason is obvious if you think about it: These companies can already see how climate disruption is affecting their operations. How will you sell coffee if there's no place to grow it? How will you manufacture soft drinks if you can't find the water? How will you serve guacamole if you can't get the avocados?

That's why Starbucks has set goals of reducing its energy consumption by 25 percent and of covering 100 percent of its electricity consumption with renewable energy by 2015. And just last week, Mars, Inc., the maker of Snickers and Uncle Ben's rice, announced that it will partner with Sumitomo Corporation of America on a wind farm in Texas that will generate more power than the company uses for all of its U.S. operations.
 
In the 21st century, it's already clear that investing in clean energy is essential for companies that want to flourish. But unlike any previous major economic shift, this time we don't have the luxury of letting things happen in their own good time. According to the International Energy Agency, our global clean energy investment needs to be about $36 trillion over the next 36 years if we want an 80 percent chance of limiting the climate warming to 3.6 degrees F.
 
Can we do it? We have to. And that means doing whatever we can to encourage not just our government (local and national) but the companies that we do business with (and that we invest in) to take action. When a company like GM, Nissan, or Tesla doubles down on electric cars, we need to support that decision, if we can, with our wallets.
 
The transition to a clean, renewable economy has already started. Our job now is to kick it into high gear.

Note: This piece first appeared on Huffington Post as part of its Earth Day celebration.

People sometimes ask me these days why the Sierra Club spends so much time taking on the fossil fuel industries and encouraging wind, solar, and other clean energy sources. Shouldn't we concentrate on saving wild places and, you know, taking more hikes?

To answer that, I like to bring up John Muir, who founded the Sierra Club in 1892. Although he died almost 100 years ago -- long before the first Earth Day -- Muir would have loved the idea of a day dedicated to celebrating (and protecting) our amazing planet. He is often remembered for opposing the destruction of wilderness, but Muir, first and foremost, was a celebrant of wilderness. Just pick up one of his books. His enthusiasm explodes off the page. His campaigns to protect the places he loved were fueled by this inexhaustible passion -- and by his conviction that experiencing the natural world could lift any human spirit as it had his own.

At the Sierra Club, we've never stopped believing in the importance of protecting those wild places, and we've spent more than 100 years continuing the mission John Muir started. I've seen wilderness work its healing power on the lives of everyone from returning veterans to city  kids who have never before seen the stars. I can guarantee you that, today, hundreds if not thousands of our 2.4 million members and supporters are not just hiking, they are scrambling up Rocky Mountain peaks, rafting through the Grand Canyon, trekking across the desert, and listening to Steller's jays argue in the boughs of giant sequoias.

But as the Club entered the 21st century, we realized that simply saving the places we loved wouldn't be enough. If we fail to address the threat of climate destruction, we could see much of the progress we have achieved -- John Muir's legacy -- undone.

So we did what John Muir did when he learned that his beloved Yosemite National Park was threatened: We organized. And, over the past decade, thanks to countless volunteers and the support of millions of Americans, we've succeeded in stopping many dirty, climate-polluting fossil fuel projects. We're proud of that work. But it's still not enough.

Here is where we can really learn from John Muir -- a man who loved wilderness so much that he took presidents camping and hiking to share his passion (and win their support). We cannot succeed if we define ourselves solely by the things that we're against. We must be just as effective, creative, and tenacious at identifying and establishing the positive solutions we do want to see. In other words, before we tell a policymaker what they should not do, we need to be sure we have a counterproposal for what they can do instead. I'll go even further. We need to show everyone in America what we can do instead.

That is what the Sierra Club is dedicated to doing today. We want to do for clean energy what John Muir did for wilderness 120 years ago. We want to show Americans -- show the entire world -- what clean, renewable energy can do for this planet we love so much.

There's no shortage of good news about renewable energy -- I see something exciting practically every day -- and each new development is another reason for optimism. That's what we should be shouting from the rooftops this Earth Day. Because if we don't get the message out there that we can turn away from fossil fuels and embrace clean energy on a larger scale, then who are the optimists? Think about it. If the only news that people ever hear is that carbon emissions are rising at an alarming rate, or that the effects of climate destruction are visible sooner than we thought, or that our leaders don't seem able to summon the political will to respond, well, then why should anybody have hope? In that situation, the only optimists will be the denialists. If we don't articulate a vision for a prosperous society powered by clean energy, then the only "optimistic" perspective is to deny reality and bury one's head in the sand. And that's a dangerous thing to do when the seas are rising.

So here's what I want everyone to remember this Earth Day: The world is a wonderful place. In just 90 minutes, enough sunlight strikes this planet to provide our planet's entire energy needs for one year. The contiguous United States has enough potential wind energy to provide all of our nation's electricity -- nine times over. Renewable energy has become economically competitive faster than anyone imagined just a few years ago -- in many places it is already beating all fossil fuels and nuclear power on price alone. Our progress toward a prosperous society powered by 100 percent safe, secure, and sustainable energy is unstoppable. We will get there -- the only question is how soon. The answer? The sooner the better.

Got it? Now, make like Muir and spread the word!