U.S. leadership is crucial. That is why I am encouraged by the spirit of compromise shown in the bipartisan initiative announced last week by John Kerry and Lindsey Graham. Here was a pair of U.S. senators — one Republican, the other Democratic — coming together to bridge their parties’ differences to address climate change in a spirit of genuine give-and-take.

We cannot afford another period where the United States stands on the sidelines. An engaged United States can lead the world to seal a deal to combat climate change in Copenhagen. An indecisive or insufficiently engaged United States will cause unnecessary — and ultimately unaffordable — delay in concrete strategies and policies to beat this looming challenge.

Leaders across the globe are increasingly showing the engagement and leadership we need. Last month, President Barack Obama joined more than 100 others at a climate change summit at U.N. headquarters in New York — sending a clear message of solidarity and commitment. So did the leaders of China, Japan and South Korea, all of whom pledged to promote the development of clean energy technologies and ensure that Copenhagen is a success.

Japan’s prime minister promised a 25 percent cut in greenhouse gas emissions from 1990 levels by 2020, laying down a marker for other industrialized nations. The European Union, too, has pledged to make a 30 percent reduction as part of a global agreement. Norway has announced its readiness for a 40 percent cut in emissions. Brazil has unveiled plans to substantially cut emissions from deforestation. India and China are implanting programs to curb emissions as well.

Looking forward to Copenhagen, I have four benchmarks for success:

Every country must do its utmost to reduce emissions from all major sources, including from deforestation and emissions from shipping and aviation. Developed countries must strengthen their mid-term mitigation targets, which are currently nowhere close to the cuts that the Intergovernmental Panel on Climate Change says are needed. Developing countries must slow the rise in their emissions and accelerate green growth as part of their strategies to reduce poverty.