Naish has researched and written about the possible effect of melting ice sheets in Antarctica on global sea levels due to high CO2 emissions causing warming in the Southern Ocean.

[3] Qualified as a professor in earth sciences, Naish was director of the Antarctic Research Centre at Victoria University of Wellington from 2008 until 2017, when he took up a Royal Society of New Zealand James Cook Fellowship.

Early work in this area began while he was completing his PhD in 1996 and became interested in a project in Whanganui, New Zealand that was exploring how sedimentary rocks could show the history of two and a half million years of changes in the sea level.

In September and October of 1999, Naish led a team that drilled deep holes in the Basin with the aim of confirming that "the Wanganui succession is gaining a reputation as one of the most complete shallow marine records of late Neogene sea level and climatic change in the world".

The aim was to reduce the uncertainty of a previous estimate that sea levels were 20 metres higher in the Pliocene period - a time when the world had about the same amount of greenhouse gases such as CO2 as in the 21st century and was a few degrees warmer.

They used a new method of analysing marine geological sediment archived from the previous Whanganui Basin research and were able to show that "during the past warm period of the Pliocene about three million years ago, global sea levels regularly fluctuated between 5 to 25 metres".

[22] This report noted that the research had discovered single-celled algae (fossil diatoms) under the ocean floor, indicating large areas of the ice shelf had retreated, possibly onto the Antarctic landmass.

[22][23] In 2007 Naish also co-authored another summary of the achievements of ANDRILL in the project's first year, confirming that the cores contained rocks with historical evidence of climate variations in Antarctica, enabling predictions to be made about how the iced sheets could respond to global warming.

[24] Overseas studies conducted on Russian sediment cores, discussed by Julie Brigham-Grette and Steve Petsch[25] from the University of Massachusetts, Amherst in 2020, confirmed that if the CO2 emission levels were not reduced globally, the fears of some scientists that the Arctic will be completely ice-free within the next two decades will be realised.

Along with Richard Levy, Naish leads the NZ SeaRise project, a five-year (2018–2023) research programme funded by the Ministry of Business, Innovation and Employment and hosted at Victoria University of Wellington.

[38]Naish was one of a team of geologists, glaciologists and social scientists that won the 2019 Prime Minister's Science Prize[39] for their work on the impact of Antarctica's ice sheets melting on global sea levels.

Naish was the team leader and another member, Richard Levy from GNS Science and Victoria University of Wellington credited him with "bringing together scientists and experts to ensure the project's success".

By 2019 Naish was confident that, based on scientific evidence, if greenhouse gas emissions were not in line with the Paris Agreement target of 2 degrees warming, both the East and West Antarctic ice sheets could be lost.

[46] Naish's presentation focused on the rise in global sea levels as a result of the loss of Antarctic ice and stressed that the science indicated this was a climate emergency that parliamentarians needed to address urgently.

[48] At end of the Assembly, all parliamentarians signed a statement of support for their parliaments to "adopt, where appropriate, additional national legislation contributing to the full and effective implementation of the Antarctic Treaty System".