The Arctic Ocean could experience ice-free summers within the next 20 years, much earlier than previously predicted. A new study has warned that unless human greenhouse gas emissions are greatly reduced, the Arctic will be nearly free of sea ice during the summer by the middle of this century.&nbsp;<br />” <br />”In addition to the human-caused warming, the melting of ice may also occur due to a natural, long-term warming phase in the tropical Pacific. Ocean temperatures in the Pacific always vary from month-to-month and from year-to-year, but slowly evolving ocean processes cause long-term temperature shifts lasting between 10 and 30 years.&nbsp;<br />”<br />”<br />”These shifts in temperature, known as the Interdecadal Pacific Oscillation (IPO), translate into an approximately 0.5 degree Celsius shift in ocean surface temperature in the tropics.&nbsp;<br />” <br />”<span style="color: #222222;">The researchers used a computer climate model to predict climate change. The study was published in the AGU journal Geophysical Research Letters.</span><br />