( CNN ) -- Record-breaking cold across the Midwest . Record-breaking heat in California . Makes you wonder what 's up with Ol ' Man Winter . For example , West Coast heat made news all week -- for better or worse . California saw a record-setting winter heat wave , but on Friday , Gov . Jerry Brown delivered the bad news : He declared a drought emergency , after seeing little rain since 2012 . That announcement came two days after Monterey , California , reached 83 degrees Wednesday , smashing a record for the date by 8 degrees . At the other extreme , it was just last week that a polar vortex delivered some of the coldest temperatures in two decades to the Midwest and Northeast . Have Mother Nature and Ol ' Man Winter gone Jekyl-and-Hyde on us ? As it turns out , there 's nothing sinister at work . It 's natural variability , meteorologists say . `` It happens every year , it just varies in intensity , '' said Taylor Ward , a CNN weather producer . Last year in mid-January , many Americans were wondering why it was so warm compared to most years . But now it 's the opposite -- well , for much of the country . It depends on where you live in relation to the jet stream . The jet stream is a ribbon of wind that flows west to east high across the globe , typically at 110 mph , although its speed can often top 200 mph . Think of it as an aerial highway along which storms travel along . But the jet stream fluctuates north and south . It 's always wobbling . It drives our temperature variability daily . One day you might be below a ridge in the jet stream and its warmth , and days later , a trough or dip will send brrrrr-utality your way . The National Oceanic and Atmospheric Administration says climatologists in recent years observe a more wavy pattern to the jet stream , bringing outbreaks of colder weather to the areas of the Northern Hemisphere -- including the United States and Europe . `` Whether this is normal randomness or related to the significant climate changes occurring in the Arctic is not entirely clear , especially when considering individual events , '' NOAA says on its climate.gov website . It 's something CNN Senior Meteorologist Brandon Miller talked about last week when asked to explain the polar vortex that was leading to the coldest temperatures in many states in decades . When asked if it was a side effect of global warming , his response was , it 's under review . `` This is a hotly researched topic . In short , yes , it could be , '' he said . `` It seems counter-intuitive that global warming could cause significant cold snaps like this one , but some research shows that it could . `` We know that different types of extreme weather can result from the overall warming of the planet , melting of the Arctic Sea ice , etc . This includes extreme distortions of the jet stream , which can cause heat waves in summer and cold snaps in winter , '' he said . J. Marshall Shepherd , the president of the American Meteorological Society , says more study is needed on whether Arctic warming will have a big effect on northern climates . In nature , air north of the jet stream is cold , and to the south it is warmer . But the Arctic is getting warmer . `` We 're relaxing or loosening that difference in temperature and that changes the jet stream and loosens it some , '' he told viewers of a White House video chat last week . `` I think this is an area we have to look ( at ) ... closely in the science . I think it is quite plausible . '' He agreed it 's too early to say with certainty . But a study by Elizabeth Barnes of Colorado State University 's department of atmospheric science found no support for the increased temperatures in the Arctic leading to additional slow-moving weather patterns . She described the relationship between the Arctic and the weather to its south as `` complex . '' In the study , published last August , she said there are likely influences from other parts of the world and internal variability . As scientists press ahead in their research , this month 's jet stream reached far down to the Gulf Coast , bringing such frigidity that Midwesterners resorted to outlandish experiments , such as tossing a cup of boiled water into the outdoor air and watching it turn to snow . Finally this week , a weather ridge in the West Coast pushed the jet stream back into Canada , to the relief of many Americans . CNN 's Michael Martinez contributed to this report .
Scientists ca n't for certainty whether global warming is behind wacky weather . Comment: But the Midwest and Northeast see coldest temperatures in decades this month . Have Mother Nature and Ol ' Man Winter gone Jekyll-and-Hyde on us ?