As global temperatures in this super El Nino year march ever higher, there's one certainty in the chancy world of weather prediction: 2015 will be the hottest year ever recorded.

As tens of thousands of leaders and delegates prepare to descend on terror-struck Paris for a global summit to fight climate change, the world's leading meteorological agencies are reporting an unwavering tale of towering temperatures.

As the US National Oceanic and Atmospheric Administration (NOAA) reported overnight, last month was not just the hottest October for global sea- and land-surface temperatures, it was also the most abnormally hot of all the 1630 months it had tracked since 1880.

So far this year, eight of the first 10 months have been the hottest on record for the respective month, and the other two laggards came second (January) and third (April).

NASA's Goddard Institute for Space Studies says its data showed global temperatures last month were 1.04 degrees above their 1951-1980 baseline - the first time the increase had cracked the 1-degree mark.

Added to the remarkable warmth so far this year, there is virtually no chance 2015 won't be the warmest year on record, Gavin Schmidt, the Goddard director, said on Twitter:

Accelerating warmth

According to NOAA, combined land- and sea-surface temperatures last month were 0.98 degrees above the 20th-century average, beating all previous Octobers. They also exceeded the record anomaly - set just one month earlier in September - by 0.07 degrees.

While that discrepancy doesn't look a lot, a chart supplied by NOAA (see below) shows most previous record margins have been much closer to 0.01 degrees, underscoring how much temperatures spurted higher in October:

Australia sizzles

As southern Australia sizzles through a November heatwave, the country was also a major contributor to the world's abnormal warmth last month.

A significant heatwave in early October scorched most of southern Australia over the first 10 days of the month, setting up the record heat.

Victoria, for instance, was almost 6 degrees warmer than average for October for mean maximum temperatures, the biggest departure from the norm for any Australian state by a remarkable 0.81-degree margin.

The record heat extended over most of the Indian Ocean, but also the central and eastern equatorial Pacific where one of the strongest El Ninos ever recorded has taken hold, as the following NOAA chart shows:

Climate signal

El Nino years typically add 0.1-0.2 degrees to average global temperatures because altered wind patterns mean the Pacific Ocean tends to absorb less heat.

With rising greenhouse gas emissions trapping more of the sun's radiation, any slowdown in the oceans' take-up of the heat means more of it is left in the atmosphere to push the mercury higher, climatologists say.

An NOAA chart (see below) shows how monthly temperatures in the central Pacific El Nino base (dubbed Nino 3.4) tend to spike during El Nino periods (in red) while falling away during the reverse event - La Ninas (blue).

The background warming from climate change is evident. The years of 2014, 2005, 2010 - and almost certainly 2015 - were all hotter globally than 1998 which included the strongest El Nino on record.

Chart topper

The reason that Dr Schmidt can be so sure that 2015 will be the hottest year on record is perhaps best shown in the chart below, also from NOAA.

The heavy black line at the top of the graph shows how the first 10 months of 2015 are tracking against the previous six hottest years.

Short of a massive volcanic eruption, meteor strike or a "nuclear winter" triggered by war, there is little reason to expect the gap to narrow.

It may even widen depending on how the current El Nino develops. NOAA, Australia's Bureau of Meteorology and others are expecting the event - as measured by temperature anomalies in the Nino 3.4 region - to peak late this year or early in 2016.

Even if it falls short of the 1997-98 event, it is still likely to nudge temperature records in November and December. National leaders, gathering in Paris to discuss curbing carbon emissions in a bid to avoid dangerous climate change, may want to keep that mind.