The most significant factor for the increase in temperature is human activity, followed by the appearance of La Ni?a in 2017, which also led to a gradual increase in temperature. So there are two main reasons that make 2017 the hottest year ever:

Long-term impact of human activities

La Ni?a phenomenon

According to the British Daily Mail reported on Jan. 18, a new study found that in the absence of El Ni?o phenomenon, 2017 became the hottest year in history.

In years when El Ni?o occurs, the direction of the prevailing winds usually changes, leading to warmer water over large areas of the Pacific Ocean, resulting in higher global temperatures. Temperatures increased in 2016 compared with the year of El Ni?o, but 2017 saw the largest increase and has become the hottest year on record.

Scientists say the most significant factor in the increase in temperatures over the past 150 years has been human activity. This includes the burning of fossil fuels, which emits large amounts of heat-trapping greenhouse gases into the atmosphere. Scientists say human-induced climate change is outpacing nature itself. World leaders should focus on the scale and urgency of the risks posed by climate change, according to the 2017 temperature record.

Experts at the UK's National Meteorological Office Hadley Center and the University of East Anglia's Climate Research Center participated in the study and produced the "Hadcrut4" dataset, which is used to assess global temperatures. The results found that temperature levels in 2017 were 1°C (1.8°F) above pre-industrial levels of 1850-1900, and 0.38°C (0.78°F) above the 1981-2010 average. 2017 became the hottest year on record.

A series of international analyses of data from the U.S. National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (Nasa) also show that 2017 became the hottest year on record.2017 temperatures exceeded the highest temperatures recorded in 2016, and some data were higher than those analyzed in 2015.

Both 2015 and 2016 saw El Ni?o, a natural phenomenon in the Pacific Ocean that increases temperatures in addition to human-caused global warming.

Dr. Colin Morice of the UK's National Meteorological Office's Hadley Centre said the 2017 global temperature data measured by the center was consistent with what other centers around the world have measured, and that without El Ni?o's influence, 2017 had become the warmest year in three years and the hottest year since 1850. 2015, 2016 and 2017 have become the three hottest years. In addition to ongoing greenhouse gas emissions, 2015 and 2016 were impacted by a strong El Ni?o. However, 2017 was the most noteworthy because El Ni?o did not occur that year; instead, La Ni?a was produced, but temperatures continued to rise.

The persistent El Ni?o of 2015-2016 increased the average annual temperature in 2016 by about 0.2°C (0.36°F), an increase of about 1.1°C (2°F) over the 1850-1900 average.

The UK Met Office's 2017 global annual mean temperature forecast shows that the global mean temperature for 2017 is expected to exceed the long-term average, remaining between 0.32°C (0.57°F) and 0.56°C (1°F). According to the average of the three major global temperature datasets, the provisional figure for 2017 is 0.42°C (0.75°F) above the long-term average, which is within the forecast range.The year-end 2016 forecast correctly analyzed that 2017 would be one of the hottest years on record.

The "HadCRUT4" global temperature dataset is compiled from thousands of temperature measurements around the world. The regional variations in temperature themselves provide information to inform understanding of the mechanisms causing warming in response to the continued accumulation of greenhouse gases in the atmosphere. Professor Tim Osborne, Director of Research at the University of East Anglia's Climate Research Unit, added that it is not only the global average temperature data that needs to be analyzed, but also the geographical patterns of warming that need to be explained. Whereas initially the understanding of climate physics was that the greater the warming in the terrestrial and Arctic regions, the smaller the warming in the sub-polar regions, that result has now been observed.

The World Meteorological Organization (WMO) has assembled five major international datasets and says temperatures have been rising for a long time. Petteri Taalas, secretary-general of the WMO, said the long-term temperature trend was more important than the ranking of individual years, and that the trend was set to rise. Seventeen of the 18 hottest years on record have occurred this century, while the past three years have been exceptional in terms of warming. Changes in Arctic warmth have been particularly dramatic, and will have a profound effect on sea levels and weather conditions in other parts of the world.

It also said that many countries around the world have been affected by extreme weather as a direct result of 2017's warmer temperatures. The United States has already seen its worst year due to the impact of extreme weather and climate disasters, while other countries have seen their economies slow down or reverse growth due to tropical cyclones, floods and droughts.

U.S. President Donald Trump has announced that the United States plans to withdraw from the Paris Agreement, the world's first comprehensive agreement on reducing greenhouse gas emissions, leaving the U.S. as the only country not to have signed the deal.

Bob Ward, a climate change expert at the London School of Economics and Political Science, said that although he was not involved in the study, extreme weather, such as hurricanes in the Caribbean and the United States, will occur around the world as temperatures rise. All countries will face the impacts of climate change. This year, the Government will begin to assess the gap between the reduction of greenhouse gas emissions and the goals of the Paris Agreement. World leaders should focus on the magnitude and urgency of the risk of global climate change due to population, wealth and climate change based on recorded temperature data.

Calculations are fraught with uncertainty because of the incomplete coverage of the globe by observations, especially in the polar regions, and limitations in the measurement equipment used to generate the data sets. The data available for the various assessments of variability are also largely derived from sporadic data in the polar regions.