First, the good ol' USA comprises 1.9% of the Earth's total surface area (9,629,091 km2 for all 50 states versus 510,072,000 km2 for the entire planet). While you wouldn't know it from such sources as Fox News or talk radio, weather patterns in the US hardly make a difference to world climate. For instance, in 2014, the eastern US was colder than the 1981-2010 average, as shown by the UAH satellite temperature data for the entire year.
|Figure 1. Annual average UAH satellite temperature anomaly for 2014.|
|Table 1. Top five hottest years in the UAH satellite record|
|Rank||Year||Temperature anomaly (ºC)|
How can that be if the US was colder than normal? Simple. The rest of the world (98.1% of the planet's surface) was generally warmer than normal—and far more of the planet was warmer than normal than was colder than normal.
Second, global warming hasn't stopped just because NOAA data shows that continental US temperatures haven't risen significantly over the past x number of years (the usual number thrown around is 20 years). Why? Take a look. Here is NOAA temperature data for both the contiguous US and the entire globe since 1990.
|Figure 2. NOAA data for contiguous US and global temperatures 1990-2014. Dashed lines represent 95% confidence intervals.|
The rate of rise is nearly the same: +0.136ºC per decade for US temperatures versus 0.134ºC per decade for the entire planet. However, the US temperature rise is not statistically significant (p = 0.308) whereas the global temperature rise is (p = 0.0000182). The difference is the variance in the data. The US data is more variable than the global data, leading to a larger standard error (± 0.130 for the US versus ± 0.025 for the globe).
The fact that the US temperature increase is not statistically significant does not mean that there is no increase in temperatures. The increase is there. What it means is that the observed increase could be due to chance. However, that fact about US temperatures has no bearing whatsoever on whether or not global temperatures have increased.
The main conclusion from this is that the US, at 1.9% of the planet's surface, does not represent the entire planet. Just because there may be a cold spell in the US does not mean that global warming has stopped. As for the reasons why some parts of the Northern Hemisphere are roasting in record highs whereas others are freezing with record lows, the short answer, I wrote a post in January 2014 on that topic.