July 2015 was a month like no other.
The three agencies with the most extensive global temperature records dating back more than 100 years, NOAA, NASA, and the Japan Meteorological Agency (JMA), all recently published data indicating July 2015 was not only the warmest July on record, but the warmest month ever recorded.
Source: NOAA
How warm was it?
According to NOAA, July 2015 was 0.81°C (1.46°F) above the 20th century average of 15.8°C (60.4°F).
This may not sound like much, but July is only one in a string of recent months that have been warmer than usual. The average global temperature for February, March, May, and June all broke their respective records. January was the second warmest January on record, April the third warmest.
Source: NOAA
The oceans have continued to warm, too. The July global average surface ocean temperature was 0.75°C (1.35°F) above the 20th century average. This, like the average global temperature, was the highest ever recorded. In fact, the 10 warmest departures from global average surface ocean temperature have all occurred within the previous 16 months.
All of this heat is putting 2015 on track to becoming the hottest year on record – right after a year that was the hottest on record.
Why the extreme warming?
There are two main reasons for the sudden increase in warming: a reversal of the Pacific Decadal Oscillation (PDO) and the return of El Niño.
The PDO and El Niño (ENSO) are two of a myriad of periodic climate patterns shifting over the planet at different timescales. Both the PDO and El Niño occur over the Pacific Ocean, and their respective phases alter the rate of energy (heat) exchange between the air and ocean. These phases change global weather patterns, affecting the seasonal climate across the planet.
From late 1998 to last summer, the PDO was negative. That’s when cold, ocean water deep in the eastern Pacific comes up to the surface and absorbs heat from the atmosphere, reducing the amount of heat energy in the atmosphere. A recent study by NASA climatologists Kevin Trenberth and John Fasullo suggests the negative PDO may be the cause for the “slowdown” in global surface warming rates.
Source: NOAA
This all changed in July 2014, however, when the PDO shifted to a strong positive phase. (Phases last 15 to 30 years.)
El Niño describes a decrease in the trade winds of the equatorial Pacific Ocean. These winds normally push the warm waters of the Pacific westward toward Indonesia, allowing for cooler waters to rise to the surface, similar to the effects of the negative PDO. However, as the trade winds diminish, the warmer water begins to retreat back across the central eastern Pacific. The larger swath of warmer water releases its energy into the atmosphere. This usually results in increased storms stretching from the central Pacific into in the southwestern United States and droughts and fires in Indonesia and Australia.
El Niño occurs every three to seven years to varying degrees. The previous strong El Niño (1997-1998) was also the strongest on record. Since late 2014, data from NOAA and NASA have indicated the presence of a strengthening El Niño in the Pacific. The latest outlook from NOAA suggests the current El Niño could peak by late fall/early winter as the strongest in history and linger as a strong event through spring 2016.
What does this all mean?
The rate of the Earth’s warming is not a steady, upward climb. Embedded within the trend are peaks and valleys, a result of natural climate variability. New data suggests that we may be leaving a valley and beginning a climb toward a new peak.
What’s important is that record-breaking heat last year and this year is not occurring in isolation.
The last 38 years have been warmer than the 20th century average. All of the top 10 warmest years have occurred since 1998. Taken together, these warm years demonstrate that the Earth’s climate has changed and continues to change.
Source: NOAA