That is a good question, and difficult to answer. If it turns out to be, it will be the warmest calendar year in the instrumental record, which goes back into the 19th century.
Regardless of what El Nino (ENSO) does, 2015 will be a warm year. Why? Because everything is warm and getting warmer and even if 2015 is less warm than 2014, it will be warm. There is no other possibility.
Even without the effects of El Nino, though, it is possible that 2015 will be warmer than 2014 because we see a lot of heat out there. If the present, relatively weak El Nino continues for a while, it will likely increase the chance that 2015 will be warmer than 2014. But current predictions suggest that 2014 will not only continue to have a strengthening El Nino, but El Nino conditions may either continue or repeat over 2015 and beyond. If that happens, not only is 2015 likely to be the warmest year in the instrumental record (since 1880) but 2016 may be in the running to be even warmer.
So far each month of 2015 has been very warm (see graph above) overall (the “zero” on the Y-axis of that graph represents the 20th century mean surface temperature). This month, April, is not excessively warm. Likely when April is plotted for 2015 on this graph, it will be either cooler then or around the same as last April.
Obviously we won’t know until the year is over, and given that climate change is a medium term phenomenon best measured in decades, we shouldn’t be in such a hurry to know these numbers. But, given that climate change is the existential issue of our day and the data become available month by month, we are not going to ignore the march of surface temperatures. We are going to, rather justifiably, be interested in what happens, month by month, as it happens.
At the end of the month, climate scientists such as my friend John Abraham, who is tracking global temperatures daily, will be able to produce a very good estimate of what the major data bases (such as NASA GISS, used here) will say, but those data bases won’t be officially updated until around the middle of the following month or so. So stay tuned.
Added: For those keeping track, I made a new version of the above graph. The red line represent the monthly anomaly values required (on average) for the rest of the year for 2015 to equal 2014. I also extended the Y-axis to 100 because the warmest month in the GISS database is in the 90s, just in case such a very warm month occurs. It is likely that April 2015 will not e as warm as April 2014 but it will likely be above the red line.