Last summer, Brexit shocked the world. Few global experts saw it coming. In their defense, most economic indicators didn't point to a political upheaval. Gross domestic product in the U.K. was growing at about 2%, and unemployment had dropped to 4.9%. From a data perspective, things seemed OK.

Another metric, however, showed something different happening in the U.K. -- "happiness." In the two years leading up to Brexit, Gallup found that the percentage of people who were "happy" (or "thriving") was in dramatic decline. In fact, the 15-percentage-point decline in the percentage of people rating their lives positively enough to be considered thriving was so dramatic that it remains among the largest two-year drops in Gallup's history of global tracking.

Today, the U.N. launches its next iteration of its "World Happiness Report." Popularized by previous reports, these happiness rankings have become world famous. Many people can quickly identify Denmark, Norway and Switzerland as among the happiest countries. What people don't know is how happiness is measured and why it's so important to track.

The U.N.'s happiness rankings use one data point from a massive survey known as the Gallup World Poll. That data point comes from a question that asks people in more than 150 countries to rate their lives on a scale of zero to 10 -- with zero being the worst possible life and 10 being the best possible life. The latest rankings show Syrians, Burundians and Central Africans rating their lives the worst (about a 3.0) and people in the Nordic countries rating their lives the best (7.5 on average).

While the metric is known as "life satisfaction," there is some disagreement as to whether it should actually be labeled as "happiness." Regardless of what we call the metric, it's important to watch.

Brexit is not the only global event for which happiness metrics served as a leading indicator of broad citizen discomfort. Perhaps the most prominent example was the lead-up to the Arab uprisings. Even the most trustworthy indicators such as GDP and the U.N.'s Human Development Index proved unreliable in forecasting the uprisings in Egypt, Syria, Tunisia and Bahrain. Happiness, on the other hand, told a different story. While GDP per capita grew in almost perfect linear fashion in all four Arab countries, happiness plummeted.

Similar trends are seen in the lead-up to the 2014 Euromaidan Revolution in Ukraine. Since the revolution, Ukrainians' happiness ratings have been at record lows. In fact, Ukraine is the only European country near the bottom of the happiness list, ranking 132nd.

Happiness might also determine how people vote, according to researchers at the London School of Economics. When comparing happiness data and election outcomes from 15 countries in Europe from 1973 to 2012, Professor George Ward found that "… a country's level of life satisfaction is a robust predictor of election results."

Using Gallup data from more than 150 countries, today's report again ranks the world's happiest countries. The information will be fun to consume -- especially for people who live in the world's happiest countries -- but leaders should pay close attention to the results. Measures of how people rate their lives are quickly becoming just as important as indicators of income and employment.

This year's happiest -- and unhappiest -- countries in the world are listed below.

These data are available in Gallup Analytics.

World Happiness Rankings Please imagine a ladder with steps numbered from zero at the bottom to 10 at the top. Suppose we say that the top of the ladder represents the best possible life for you, and the bottom of the ladder represents the worst possible life for you. On which step of the ladder would you say you personally feel you stand at this time, assuming that the higher the step the better you feel about your life, and the lower the step the worse you feel about it? Which step comes closest to the way you feel? Life Today 2014-2016 Norway 7.537 Denmark 7.522 Iceland 7.504 Switzerland 7.494 Finland 7.469 Netherlands 7.377 Canada 7.316 New Zealand 7.314 Australia 7.284 Sweden 7.284 Israel 7.213 Costa Rica 7.079 Austria 7.006 United States 6.993 Ireland 6.977 Germany 6.951 Belgium 6.891 Luxembourg 6.863 United Kingdom 6.714 Chile 6.652 United Arab Emirates 6.648 Brazil 6.635 Czech Republic 6.609 Argentina 6.599 Mexico 6.578 Singapore 6.572 Malta 6.527 Uruguay 6.454 Guatemala 6.454 Panama 6.452 France 6.442 Thailand 6.424 Taiwan 6.422 Spain 6.403 Qatar 6.375 Colombia 6.357 Saudi Arabia 6.344 Kuwait 6.105 Slovakia 6.098 Bahrain 6.087 Malaysia 6.084 Nicaragua 6.071 Ecuador 6.008 El Salvador 6.003 Poland 5.973 Uzbekistan 5.971 Italy 5.964 Russia 5.963 Belize 5.956 Japan 5.920 Lithuania 5.902 Algeria 5.872 Latvia 5.850 South Korea 5.838 Moldova 5.838 Romania 5.825 Bolivia 5.823 Turkmenistan 5.822 Kazakhstan 5.819 Northern Cyprus 5.810 Slovenia 5.758 Peru 5.715 Mauritius 5.629 Cyprus 5.621 Estonia 5.611 Belarus 5.569 Libya 5.525 Turkey 5.500 Paraguay 5.493 Hong Kong 5.472 Philippines 5.430 Serbia 5.395 Jordan 5.336 Hungary 5.324 Jamaica 5.311 Croatia 5.293 Kosovo 5.279 China 5.273 Pakistan 5.269 Indonesia 5.262 Venezuela 5.250 Montenegro 5.237 Morocco 5.235 Azerbaijan 5.234 Dominican Republic 5.230 Greece 5.227 Lebanon 5.225 Portugal 5.195 Bosnia and Herzegovina 5.182 Honduras 5.181 Macedonia 5.175 Somalia 5.151 Vietnam 5.074 Nigeria 5.074 Tajikistan 5.041 Bhutan 5.011 Kyrgyzstan 5.004 Nepal 4.962 Mongolia 4.955 South Africa 4.829 Tunisia 4.805 Palestinian Territories 4.775 Egypt 4.735 Bulgaria 4.714 Sierra Leone 4.709 Cameroon 4.695 Iran 4.692 Albania 4.644 Bangladesh 4.608 Namibia 4.574 Kenya 4.553 Mozambique 4.550 Myanmar 4.545 Senegal 4.535 Zambia 4.514 Iraq 4.497 Gabon 4.465 Ethiopia 4.460 Sri Lanka 4.440 Armenia 4.376 India 4.315 Mauritania 4.292 Congo (Brazzaville) 4.291 Georgia 4.286 Congo (Kinshasa) 4.280 Mali 4.190 Ivory Coast 4.180 Cambodia 4.168 Sudan 4.139 Ghana 4.120 Ukraine 4.096 Uganda 4.081 Burkina Faso 4.032 Niger 4.028 Malawi 3.970 Chad 3.936 Zimbabwe 3.875 Lesotho 3.808 Angola 3.795 Afghanistan 3.794 Botswana 3.766 Benin 3.657 Madagascar 3.644 Haiti 3.603 Yemen 3.593 South Sudan 3.591 Liberia 3.533 Guinea 3.507 Togo 3.495 Rwanda 3.471 Syria 3.462 Tanzania 3.349 Burundi 2.905 Central African Republic 2.693 Averages based on Gallup World Poll surveys conducted in 2014-2016 Gallup World Poll