Mrs. Begum’s biggest challenge is not what the sea level may do in five or 10 decades. She has a more modest request: “It would be a heaven’s gift if a proper drainage system could be arranged in this area where all the drains are covered and do not overflow.”

Getting basic sanitation and safe drinking water to the three billion people around the world who do not have it now would cost nearly $4 billion a year. By contrast, cuts in global carbon emissions that aim to limit global temperature increases to less than two degrees Celsius over the next century would cost $40 trillion a year by 2100. These cuts will do nothing to increase the number of people with access to clean drinking water and sanitation. Cutting carbon emissions will likely increase water scarcity, because global warming is expected to increase average rainfall levels around the world.

For Mrs. Begum, the choice is simple. After global warming was explained to her, she said: “When my kids haven’t got enough to eat, I don’t think global warming will be an issue I will be thinking about.”

— Bjørn Lomborg, Global Warming as Seen From Bangladesh,