Investing.com - An escalation of trade tensions between the U.S. and other major countries is the biggest threat to the global economy, according to the International Monetary Fund's latest economic forecast.The IMF's World Economic Outlook report said additional tariffs by the U.S., China and others could knock half of one percent off of global growth.Despite that warning, the IMF did not change its forecast for global growth for 2018. It still expects the global economy to grow by 3.9%. Forecasts for China and the U.S. were also unchanged.It did, however, lower its GDP growth forecasts for the euro zone, Japan and the U.K. in 2018.