What did you study in school?

I got interested in politics. I began to understand how British politics had come out the way it had. I was on the left, and we kept on losing. Why was that? And I sort of came to the conclusion that, if you can make a difference, you should. And if you don’t, it’s a waste.

You had several different roles in politics. What policy decision that you were involved in do you feel was most consequential?

I was involved in writing the manifesto for the ’97 general election. That was probably the most significant document I’ve ever been involved in writing. We had to ditch a load of bad policy and invent a load of good policy, which could both persuade voters and change the country. I think that governments actually have more impact over the medium term than they realize. We changed the country in quite fundamental ways. Socially, economically and in matters of peace and war in Northern Ireland. I honor people who are in politics, on whatever side. In my experience, the vast majority of M.P.s are not charlatans.

You also served as environmental secretary, where you focused on climate change.

In 2006 — I mean, those were the halcyon days before the financial crisis — we launched the Climate Change Bill, which became the Climate Change Act. And to American ears it may sound extraordinary, but we set out to bind every future administration between 2006 and 2050 with a long-term commitment to reduce carbon emissions; originally by 60 percent, then by 80 percent. It was a robust piece of legislation. Obviously, if your economy is doing badly — and the U.K. is doing badly at the moment — it’s easier to meet your carbon targets. But I think that this is the overwhelming generational challenge.

What precipitated your exit from politics?

I lost two elections in 2010. One was a general election, which my party lost. One was a leadership election, which I lost to my brother. I was still a member of Parliament, but I was presented with a choice of silence or division. I could either remain silent and sort of violate the basic purpose of a politician, which is to persuade people. Or, if I spoke out, I’d just be divisive and play into a soap opera.

And why did you join the I.R.C.?

I remember saying to the hiring committee that I wanted the job here for three reasons, not just because I was sort of being pushed out of the U.K. One was that issues at the border are some of the most difficult questions in global public policy. How do you get medical aid into Syria? How do you teach girls in Afghanistan? How do you tackle sexual violence in Congo? Nevermind fight Ebola in Congo, which we’re doing at the moment.

Second, that the I.R.C. should not be a sleeping giant, it should be a roused leader of the global humanitarian sector. It had a responsibility to step up to a global leadership role.