Over the past few years something remarkable has happened to New York and you can only really appreciate it from outer space.
If you compare a satellite picture of New York in 2009 and 2013, you'd see that the roofs have turned white.
Even more impressively, it's been done entirely by volunteers.
It's all thanks to NYC Cool Roofs which has painted almost four million square feet of New York's roofs with white reflective paint, savings tens of thousands of tons of carbon emissions because it means people dial down the aircon.
It's just one initiative from New York's pioneering City of Service programme which has mobilised New Yorkers to give their time, passion and skills in the service of others.
I first heard about NYC Service two years ago when I met Diahann Billings-Burford, America's first City Service Officer and one of the people who has defined the concept of impact volunteering.
I'll confess to being a little bit in love with Diahann. With a background in AmeriCorps and City Year, she is a powerful advocate of service. But she's also clear that too many initiatives calling on voluntary effort fail to make an impact and is ruthless in stopping programmes that can't provide evidence to back up their claims.
If you are using people's generosity and time to teach kids to read, then let's see the numbers. How quickly are those kids advancing their reading? Are your volunteers using the best methods? If not, fix it or stop wasting people's time.
Working through a partnership spearheaded by his foundation, Mayor Bloomberg has taken the idea national and 162 American Cities now have their own Service strategies built around the idea that voluntary effort should be targeted at a measurable impact.
There are even blueprints and playbooks that codify the best methods based on evidence of what works.
The key to impact volunteering is in knowing what impact you're trying to achieve and focusing relentlessly on that goal. Just because people give their time for free doesn't mean it can be wasted.
In a deliberate attempt to learn from the Cities of Service movement, one of our first themes for the Centre for Social Action Innovation Fund is "impact volunteering" and we're on the lookout for programmes in England that we can invest in and help to grow.
The best example I've seen (so far) is happening at Kings College Hospital - the Kings Volunteers.*
It started with the simple idea of asking staff what more they would like to do to help patients but couldn't because they didn't have the time.
That generated a list of simple, humane acts of kindness, from welcoming and guiding people around the building, to running errands, talking to patients and holding their hands while they wait for surgery.
The hospital turned those into volunteer role descriptions and asked local people to help out. They responded in their thousands and the hospital now routinely deploys over 700 volunteers from diverse backgrounds who have all committed to regular volunteering over a year.
Kings is already seeing an increase in patient satisfaction and the next stage is to track the effect on discharge, recovery and wellbeing.
If we can show a positive impact on those metrics and codify how they achieved it, then our plan is to offer it as a model to other hospitals and adapt it for other public services.
Kings has established a model that relies of a high degree of commitment from the volunteers. That isn't for everyone (commitment is something I'll be covering in a future blog) but Cool Roofs shows us that episodic low-commitment volunteering can be just as impactful if the opportunity is designed right.
I've no doubt that there will be other examples on this side of the Atlantic that already reflect the spirit of impact volunteering. If so, we want to hear from you.
@philipcolligan is the Executive Director of Nesta's Innovation Lab and Government Adviser on Social Innovation
*Nesta and the Cabinet Office have supported the development of the Kings Volunteers scheme with a grant of £181,000 from the Innovation in Giving Fund