If I were to write a letter to a congressman about climate change, I would focus on pushing for renewable energy.
Explain that in today's society we are going to need energy. However, as we continue to rely on fossil fuels, we are polluting the atmosphere with more and more carbon which has gradually lead to higher global temperatures. To help decrease the amount of carbon emissions, we should focus on funding and turning to renewable energy.
In order for our nation to change from fossil fuels to more renewable resources (such as solar, hydroelectric, wind, and nuclear energy), congressmen need to write, vote for, and pass bills mandating it. Urge the congreeman in your letter to be vocal about making these changes and encourage their fellow congressmen and women to make these changes as well.