Globally, 2018's temperatures ranked behind those of 2016, 2017 and 2015.
Continuing a trend of rising temperatures, 2018 was the fourth hottest year on record, according to a climate change report out Wednesday from NASA and the National Oceanic and Atmospheric Administration (NOAA).
"The impacts of long-term global warming are already being felt - in coastal flooding, heat waves, intense precipitation and ecosystem change".
That means 2018 was the fourth hottest year recorded in over 100 years.
In 2015, nearly 200 governments adopted the Paris climate agreement to phase out the use of fossil fuels and limit the rise in temperatures between 1.5C to 2C, to avert "dangerous" man-made climate change.
NOAA and NASA contribute data to the WMO.
Moreover, 2015 was the first year when global annual average surface temperatures reached 1C above pre-industrial levels, and the following three years have all remained close to this level. That would be warmer than the last four years.
Schmidt also said that increasing temperatures can also contribute to longer fire seasons and some extreme weather events.
Schmidt also stated he was "very concerned with what is going on in the Arctic" as it is heating up at twice the average rate of the rest of the world, and 2018 saw the average extent of sea ice at its lowest level.
"That is not saying the Paris Agreement is done for. but it's a worrying sign", he added. That puts us more than two-thirds of the way to the warming limit of 1.5 degrees Celsius that was set in the Paris climate agreement.
Patrick Verkooijen, head of the Global Center on Adaptation in the Netherlands, told Reuters that the WMO report showed "climate change is not a distant phenomenon but is here right now".