With burning of fossil fuels resulting in increased CO2 concentration, I get how increase in CO2 leads to a hotter planet and global warming because Earth's heat balance has been affected by CO2 interfering with heat reflectivity. But how does a hotter planet lead to lower localised extreme cold conditions at times - eg: snow in UK and USA last winter
The increase in carbon dioxide is altering climate, with an overall increase in temperature. This does not mean that change will be uniform or consistently unidirectional.
On a daily basis we experience weather, not climate and weather can be highly variable. It's with hindsight that we can state that the season just gone was average (typical) or extreme, or somewhere in between. Depending on location, one week can have blazing temperatures, followed by a week of near freezing temperatures.
Factors that come into play are:
- What's the situation with El Nino & La Nina in the Pacific Ocean?
- What's the situation with the Indian Ocean equivalent, the Indian Ocean Dipole?
- What is happening with the jet streams? Are they meandering more than usual?
- How are localized high and low pressure systems bringing in cold air from the Arctic or Antarctic regions, or warm from the equatorial regions? Are there cold or warm fronts in the region and how do they combine with the localized pressure systems to affect weather locally?
- Particularly for the northern hemisphere, the behavior of the polar vortex and how a weakening polar vortex can increase the meandering of the jet stream, allowing tongues of cold air to react part of the the UK and the US that normally would not be affected by such conditions. (Thanks to @David Hammen for reminding me of this).
It's all part of the variability of weather: two steps forward, one step backwards, but the overall trend is towards global warming.
The impacts of global warming on weather patterns are complex, but include the continuing potential for extreme weather events at local and regional scales, including extreme cold events from polar air being driven to lower latitudes.
Weather patterns that cause cold air masses to move to lower latitudes include disruptions to the Polar Vortex. The Polar Vortex is a strong pole circling stratospheric wind pattern that occurs in Winter; it tends to keep cold Arctic air close to the pole. When it is disrupted the winds weaken and meander North and South more causing both extreme heat and extreme cold events.
Why the Polar Vortex gets disrupted is still being studied but it is hypothesized that Polar Amplification, where loss of Arctic sea ice causes more heat to be absorbed by Arctic waters and causes heat to persist longer and Arctic air temperatures to rise more than the global average plays a part and is increasing the frequency of Polar Vortex disruption. It is thought the mechanism is that this warmer than normal (but still very cold) Arctic air rises and disrupts the Polar Vortex, which in turn allows cold Arctic air masses to spill further South than usual and warm air to spill further North.