Vietnam produces more than a third of the world's robusta coffee – but drought threatens production and prices.Nhac Nguyen/AFP via Getty ImageRobusta coffee futures hit a 16-year high because of a prolonged drought in Vietnam.Vietnam and Brazil, the top robusta producers, face climate issues affecting harvest and supply.Vietnamese farmers are also holding onto their beans, hoping to sell them for higher prices.Lattes may get pricier, as a key indicator for the price of the bean in espresso just hit a record high.The robusta coffee variety is the basis for both espresso and instant coffee.