Agence France-Presse: Top climate envoys Tuesday said they were more optimistic about sealing a global warming deal this year after the two-day 18-nation talks here, but some participants said countries were still not making firm commitments. US President Barack Obama, who champions more aggressive action against global warming, invited negotiators from 17 other major economies including developing powers such as China and India to meet in Washington. The talks came as the clock ticks to a December …