With a direct letter sent to nations, host country Brazil is shifting the U.N. climate conference into a higher gear. The letter sent late Monday comes during the final week of what has been billed as a historic climate summit , the first ever in the Amazon rainforest, a key regulator of climate because trees absorb carbon dioxide, a greenhouse gas that warms the planet. The letter comes ahead of speeches of high-level ministers Tuesday. Headliners include representatives from influential