A wide variety of weather phenomena (heat and cold waves, winter storms, hurricanes, tornadoes, and flooding) display more-or-less consistent patterns of frequency and severity—but such patterns have been increasingly inconsistent during recent decades. Though natural variability plays a role in determining the course of weather systems, there is evidence that modern trends observed by scientists are driven by a unified causal factor. Isolating that causal factor, however, has proved remarkably difficult, despite substantial climatological research. The types of evidence analysis that normally yield understandings of broader climate phenomena, like glacial core sampling or the observation of tree rings, have been tricky to apply in analyzing more complex relationships between individual weather events. Moreover, climate scientists and meteorologists have to distinguish weather anomalies from weather events representative of relevant trends.
Nonetheless, detailed research indicates that the primary causal factor behind weather trends may be anthropogenic climate change. Recent decades have seen increases in the number of days yearly that reached record high temperatures and decreases in the number of days yearly that reached record lows, but instances of record-breaking winter precipitation (snow) have increased as temperatures have risen. This information was once seen as contradicting established models of climate change, but scientists have explained it through a new hypothesis that accounts for the amount of moisture in the atmosphere. Warm periods of increased duration are postulated to cause increased winter precipitation if, evaporating vast quantities of water into the air, they dry some areas while weather systems bring the evaporated moisture to others. Once the moisture reaches colder climates, it condenses, and falls back to earth as rain or snow. As more regions become warmer for longer periods, the amount of airborne water increases, as do instances of extreme rain and snow. Thus, seemingly minor changes in global temperature can have significant effects on the frequency and severity of weather phenomena in every season and environment.
What most recommends this theory is its ultimate simplicity. Though it may seem that droughts in some areas are unrelated to periods of increased precipitation in others, climate change is responsible for an environmental continuum that produces both. Increasing temperatures worldwide is the one cause that can explain all of these disparate phenomena.