Telegraph: Never has the challenge of saving the world felt as simple as it does right now. Sitting on the sofa in front of me, Jeffrey Sachs is leaning back, gingerly sipping his coffee and sweeping away some of the most intractable problems facing our planet with the barest waft of his palm. Poverty and a billion starving people in Africa? Whisked away like dust. Overpopulation and water shortages? Waft, waft. Global warming and climate change? A little brush and they’re gone. It would …