In delivering aid to drought-stricken California last week, President Obama and his aides cited the state as an example of what could be in store for much of the rest of the country as human-caused climate change intensifies.

But in doing so, they were pushing at the boundaries of scientific knowledge about the relationship between climate change and drought. While a trend of increasing drought that may be linked to global warming has been documented in some regions, including parts of the Mediterranean and in the Southwestern United States, there is no scientific consensus yet that it is a worldwide phenomenon. Nor is there definitive evidence that it is causing California’s problems.

In fact, the most recent computer projections suggest that as the world warms, California should get wetter, not drier, in the winter, when the state gets the bulk of its precipitation. That has prompted some of the leading experts to suggest that climate change most likely had little role in causing the drought.