Forests in Europe, North America, the Caucasus, and Central Asia have expanded steadily over the last two decades, increasing by more than 25 million hectares — an area slightly larger than the United Kingdom — since 1990, a UN report says. In Europe alone, forested areas increased by 17 million hectares from 1990 to 2010, with the volume of forests growing by more than 430 million cubic meters annually, according to the Global Forest Resource Assessment 2010. Most of the forest expansion occurred in Europe, the Caucasus, and Central Asia, where areas targeted for conservation of biological diversity have increased almost 1 million hectares annually since 1990. Forest growth in these regions — and the consequent addition of about 5 billion tons of carbon storage capacity — is particularly critical since forest cover has decreased globally, particularly in tropical regions. However, the report found that climate variability poses an increased threat to forests, particularly in North America, where outbreaks of mountain pine beetles — linked to warmer winters — have devastated more than 11 million hectares since the late 1990s.