It’s been quite a year for climate-watchers. To start with, although the official numbers for all of 2010 won’t be in until early next month, global average surface temperatures so far have been at record high levels, in keeping with the recent warming trend that scientists say has very likely been caused in large part by human emissions of heat-trapping gases such as carbon dioxide (CO2).

This year also featured plenty of extreme events, from crippling snowstorms in the American Northeast to blazing heat and deadly flooding in Pakistan. Many of these events have already been at least partially linked to natural variations that occur in the Earth’s climate system. These kinds of climate oscillations, like El Niño, La Niña, and the North Atlantic Oscillation, influence weather patterns around the globe, and in 2010 all three of these were in action. It is also sometimes possible to attribute extreme events, like some of those in 2010, to long-term global climate change, but this usually takes a few years of scientific analysis.

Climate Central’s scientists, working in consultation with outside experts, have compiled our first annual “Top 10 Climate Events” list based on the events that had the greatest impacts, and which stood out most in the historical record. Here they are, in chronological order:

1. Mid-Atlantic Cities Break All-Time Snowfall Records

The year got off to a snowy start in the eastern U.S., with record-breaking storms along the Mid-Atlantic coast. The Nor’easter that struck in the first week of February — which quickly became known as “Snowmageddon” – dumped so much snow that it helped Baltimore, Wilmington, Philadelphia, Atlantic City and Washington break all-time records for winter snow totals. For example, Washington’s Reagan National Airport received 56.1 inches of snow during the 2009-10 winter, compared to the average total of just 15.2 inches!

Statistically, each of the storm systems that passed through the Mid-Atlantic and Northeast in early 2010 was quite rare. According to the National Climatic Data Center (NCDC), for example, Philadelphia should expect a 22-inch or greater snowfall only once every 100 years. Yet the winter of 2009-2010 saw two storms of that magnitude.

Of course, big snowstorms also make people wonder what ever happened to global warming. The short answer is that it didn’t go anywhere, and in fact there is some evidence that warmer global temperatures can lead to heavier precipitation events, including heavy snowfalls. In a warmer world more water evaporates from the oceans and then can fuel more intense precipitation. However, the main contributor to last winter’s extreme snowstorms was a natural climate pattern known as the North Atlantic Oscillation, or NAO, which influences winter weather in parts of the northern hemisphere.

Typically, low pressure dominates over the Arctic, and high pressure across the mid-latitudes. This pressure difference generates winds that tend to confine extremely cold air to the Arctic. But sometimes these two pressure systems weaken, decreasing the difference between the Arctic and mid-latitudes and allowing chilly Arctic air to slide south. This pattern is called a negative-NAO. When the NAO is negative, as it was to an unusually significant and prolonged degree last winter, the odds of a major snowstorm in the Mid-Atlantic and Northeast increase.

2. Flooding in Nashville, Tennessee

Spring typically brings rain showers to much of the U.S., but in 2010 excessive rainfall caused a number of serious floods in the Southeast, the worst of which occurred in Tennessee. On May 1, rain began falling heavily in the middle and western regions of the state. Within 48 hours, between 13 and 19 inches of rainhad fallen, and many rivers overflowed. When the deluge finally let up two days later, large areas had been engulfed by floodwaters. At least 33 people died in the Tennessee floods, and the Nashville Planning Department says the state’s capital suffered $1.9 billion in damages — including damage to the iconicGrande Ole Opry Theatre.

Similar but less intense floods also occurred in Oklahoma City and rural Arkansas in early June, but the kind of rainfall that caused the Tennessee floods was so rare that it qualifies as one of the most remarkable climate events of the year.

According to the National Weather Service, the early May storm was a 1 in 1000-year event, which means that in any given year, there is a 0.1 percent chance that it might occur.

3. Record-breaking Heat Waves and Droughts in Africa and the Middle East

In June 2010, a scorching heat wave in Africa and the Middle East broke a number of all-time temperature records in the region. In fact, between May and June, seven countries broke their previous high temperature records: Pakistan, Myanmar (Burma), Iraq, Saudi Arabia, Chad, Niger and Kuwait. On June 15, Kuwait posted its hottest-ever temperature when the mercury rose to 126.7°F in the city of Abdaly.

The incredible heat did more than just shatter temperature records. In Saudi Arabia, a sandstorm that accompanied the extreme heat led to a number of power-outages in parts of the country. In Pakistan, temperatures climbed as high as 128.3°F, the highest temperature ever recorded in the continent of Asia, according to Weather Underground.

4. Russian Heat Wave

Another region struck by intense heat during the summer of 2010 was western Russia, and Moscow in particular. On June 29, the mercury rose above 100°F in Moscow for the first time on record, and such abnormally high temperatures lasted for well over a month. According to Weather Underground meteorologist Jeff Masters, Moscow experienced 26 consecutive days with temperatures above 86°F this summer and then, on August 6, records broke again when Moscow hit 102°F. Elsewhere in Russia, temperatures climbed as high at 111°F, the highest temperature ever recorded in that country (not including the former Soviet Republics).

A Climate Central analysis put the high average temperature in Moscow in the 2010 summer (during June, July and August) in the context of the city’s long-term climate record using data since 1950. The analysis showed just how exceptional this past summer was in Russia.

This image shows that the average temperature in Moscow for the summer of 2010 (the months of June, July and August) was significantly hotter than any year since 1950. Credit: Climate Central

The persistent heat wave had a devastating impact on the country. Wildfires spread across western portions of Russia, and the combination of smoky air and unusually warm weather led to at least 10,000 deaths in Moscow alone. Authorities estimate that during some stretches in early August, as many as 700 people were dying each day. Furthermore, the intense heat damaged wheat crops, leading the Russian government to halt exports of that valuable food staple, causing price spikes in global grain markets.

Scientists are investigating the Russian heat wave to try to determine if global climate change was partly to blame. A 2005 study showed that longer-term climate change made similar extreme events more likely, including the deadly 2003 European heat wave, but there aren’t yet any conclusions on the connection between global warming and the 2010 Russian heat wave.

5. U.S. Summer Heat Waves

The United States was not spared from the year’s record heat events. Across the East, the summer was marked by several episodes of extreme heat and humidity. Delaware, New Jersey and North Carolina experienced their warmest June on record, as did several cities, including New York, Washington, and Philadelphia.

On July 6 and 7, an intense heat wave struck the Northeast, from Maine down to Pennsylvania. Parts of New York, New Jersey and Connecticut were hit with temperatures above 100°F. Overall, the month of July was the hottest on record in Rhode Island and Delaware and it ranked amongst the 10^th warmest for each state along the East Coast.

August was another scorcher in the U.S.. According to NCDC, which is part of the National Oceanic and Atmospheric Administration (NOAA), hundreds of cities broke their daily maximum temperature records. In fact, August was the eighth consecutive month with above average temperatures in the Northeast. The Southeast also felt the late summer heat, with 20 locations reporting their warmest-ever month of August. Among them: Gainesville, Fla., Columbus, Ga., Greenville, S.C. and Charleston, S.C..

Even though the West Coast didn’t experience the same intense heat through the summer – and in fact wasin some cases quite chilly – the period of June to August 2010 was the fourth warmest on record for the entire country.

6. Pakistan Monsoon and Flooding

The monsoon season was particularly cruel in Pakistan. In late July, unrelenting torrential rains battered the eastern part of the country, triggering severe flooding. Within just four days after the rains began, the Associated Press reported nearly a thousand people had been killed. The floods continued well into September. According to the United Nations, floodwaters drove millions of people from their homes and affected a total of 20 million people. In addition to destroying homes and livelihoods, the flooding caused extensive damage to Pakistan’s infrastructure, including power plants and thousands of miles of roads, railways, and public buildings. Labeled as the worst natural disaster in Pakistan’s history, the death toll has reached nearly 2,000.

The Asian Monsoon happens every year when cool humid air from over the Indian and Pacific Oceans washes over the land, warms and then rises, cools and condenses, causing the moisture in the air to fall as heavy rains. But in 2010, the presence of La Niña in the Pacific may have intensified the process. During La Niña years, the waters in the Western Pacific are warmer than average, and the air above the ocean is more humid. This especially humid air may have increased the amount of rain that pounded Pakistan.

7. Third Lowest Arctic Sea Ice Extent

Every summer, as the Northern Hemisphere warms up, the amount of sea ice in the Arctic begins to shrink back. It usually reaches its minimum extent in September, then starts to refreeze as temperatures begin to drop. The 2010 minimum came on September 19, 2010 — and this year’s was the third lowest ever recorded by satellites since such records began in 1979.

Average sea ice extent for September 2010 was 1.89 million square miles, about 830,000 square miles less than the average September extent between 1979 and 2000. The minimum this year, however, was still 230,000 square miles more than in 2007, which had the lowest Arctic sea ice coverage ever measured. For only the second time since satellite records began, the U.S. National Ice Center declared both the Northwest Passage above Canada and the Northern Sea Routes above Scandinavia and Russiaopen for a period in the late summer.

Thanks to satellite observations, we know Arctic sea ice has declined dramatically over at least the past thirty years, and scientists have attributed this in large part to increasing concentrations of greenhouse gases in the atmosphere from burning fossil fuels such as coal and oil.

8. Lake Mead Record Low

The Hoover Dam (originally known as the Boulder Dam) was erected in the 1930s, and by 1943 Lake Mead had risen out of the Colorado River to a height of 1,220 feet above sea level. But this year, on October 18, Lake Mead reached a record low, dropping down to just 1083.9 feet, having lost about 12 stories of height. Though still about eight feet above the designated point of a critical water shortage, the low water levels are a warning signal to the millions of people in Southwest states who rely on this resource for drinking water and irrigation.

The level of Lake Mead has been steadily falling since 2000, with the exception of a slight rise in 2005, reflecting the drought that has afflicted the American Southwest over the same period. In the past ten years, a particularly dry and warm climate has lingered in Utah, Nevada, Arizona and Southern California, leading to reduced flow along the Colorado River. In fact, scientists have alreadyshown that the stress on the water resources in the Southwest region is consistent with the effects of a warmer climate, and that increased emissions of heat-trapping gases are linked to recent changes in river flows and winter snow pack. In addition to this ongoing drought, cities that draw water from Lake Mead, like Las Vegas, have grown in recent years and are further taxing the water supply.

The drought outlook through the winter does not look encouraging, as a strong La Niña event has taken hold in the tropical Pacific Ocean. La Niña conditions, which are characterized by colder than average waters off the coast of South America and along the equator, tend to be associated with drier than average winters in the American Southwest.

9. Amazon Drought

For the second time in five years, the Amazon River basin in northwestern Brazil is in severe drought. Brought on by a particularly arid dry season through April and May, the drought has extended through to November. One of the primary Amazon tributaries, the Rio Negro, dropped to its lowest level since records began in 1902, according to the Brazilian Geological Service. As water levels along the Rio Negrodropped severely in October, water temperatures in the river also began to climb, killing millions of fish and contaminating the water supplies for thousands that live in the region.

While droughts along part of the Amazon are not very unusual (during the 20^th century they occurred about once every twelve years on average), they have typically occurred in years that featured El Niño conditions. The warmer water temperatures in the Pacific associated with El Nino tend to rob the Amazon of rainfall. However, the intense drought late this year has been unexpected since La Niña has brought cool waters to the Pacific.

10. Final Annual Temperature Ranking

The latest numbers from NOAA are in, and January to November 2010 is tied with 2005 for the dubious honor of “The Warmest Year on Record” (records date back to 1880). As for November 2010, it was officially the second warmest on record, even with the cooling effect of La Niña in the mix. The global oceans ranked only 10th warmest, whereas the land surface made up the difference and came in more than 0.3ºF warmer than the previous warmest November. The full year is expected to finish up as the hottest on record, or close behind, once the statistics for December are averaged in.

This article was originally published by the good folks at Climate Central.