Continued freak warmth in the Arctic helped propel global average temperatures to the second-warmest level on record for the month of April, NASA reported on Monday.

Separately, on Thursday the National Oceanic and Atmospheric Administration (NOAA) reported the same rank for the month, and also said the year-to-date currently ranks as the 2nd-warmest January through April period, behind the same timeframe last year.

The new findings illustrate how the planet is still setting climate milestones even in the absence of other factors that tend to elevate global air and sea temperatures, such as an El Niño event.

This makes clear the growing influence of human-caused global warming on the planet's climate, month after month, year after year, as the amounts of planet-warming greenhouse gases in the atmosphere continue to rise to levels unseen during all of human history.

According to NASA, April 2017 was 0.88 degrees Celsius, or 1.58 degrees Fahrenheit, warmer than the 1951-1980 average for the month. This came in second to April of last year, when global average temperatures were 1.06 degrees Celsius, or 1.9 degrees Fahrenheit, above the 1951 to 1980 average. An El Niño event was taking place in April of 2016, which gave an added boost to global temperatures already elevated from global warming.

The top 3 hottest months of April to occur since instrument records began in 1880 have all taken place since 2010.

Arctic sea ice age at the end of March 2017. Image: nsidc

For April of 2017, the story is dominated by unusual warmth in parts of the Arctic, including across Siberia, parts of China, Alaska and the northwest portions of the Arctic Ocean. Greenland, however, had below average temperatures for the month, though that weather pattern reversed itself in early May.

Arctic sea ice tied for for the lowest level on record during the month of April, after setting record lows throughout the fall and winter. The sea ice cover, which has been declining since satellites first began keeping tabs on it in 1979, is now far thinner and younger than average as it enters the summer melt season. (Older, thicker ice has a higher chance of surviving the summer melt.)

This raises the likelihood that there could be another record low ice extent at the end of the melt season in September.

Observed warming is greater in the Arctic & less in the southern oceans. Climate models simulate this pattern too. https://t.co/CTTJmCQfR0 pic.twitter.com/A7CJgNltvo — Climate Lab Book (@ClimateLabBook) May 13, 2017

The unusually mild (though still cold by everyday standards in lower latitudes) Arctic temperatures caps off a winter season that featured repeated pulses of warm, moisture-rich air sweeping across the Arctic from the North Atlantic. It also follows what was a record warm 2016 for the region.

Scientists are racing to deploy instruments that will monitor the Arctic environment, kicking off a United Nations-sponsored initiative called the "Year of Polar Prediction."

“The rate and implications of polar environmental change is pushing our scientific knowledge to the limits,” Thomas Jung of the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, warned in a press release announcing the program.

Arctic warming (red) compared to the global average (black). Image: noaa.gov

According to the National Oceanic and Atmospheric Administration, the average annual air temperature over Arctic land areas was the highest in the observational record in 2016, representing a 3.5-degree Celsius, or 6.3-degree Fahrenheit increase since 1900. Arctic temperatures have been increasing at about double the rate of lower latitudes, in large part due to feedbacks in the climate that kick in as sea ice and land-based snow cover melts.

Few climate scientists think that 2017 will set another record for the warmest year, as 2015 and 2016 did. However, it is still likely to rank in the top 5 or top 10 warmest years on record, which is important since even non-El-Niño years are now ranking near the top of the list.

What matters to climate researchers is not one month of temperature anomalies, or even a full year, but rather the trends over decades to centuries. There, too, the human fingerprint on the climate is readily visible, as the planet continues to head toward uncharted territory for human society.