Former Defence Force officers, including a former chief of the Australian military, have warned that climate change will emerge as the defining security threat of the 21st century and urged governments to step up their responses accordingly.

The warnings come as the Senate convenes an inquiry into the national security implications of the environmental phenomenon, called by Greens senator Scott Ludlam. The government has dismissed the move, arguing it is unnecessary because of actions already being taken by the Defence Force.

But a former Defence Force chief, retired admiral Chris Barrie, says the overall response to the "existential threat" needs to be ramped up as Australia faces particular exposure to the consequences of extreme weather events, higher temperatures and sea level rises.

Mr Barrie said the world's governments were not on track to keep warming to 2 degrees celsius – the target laid out in the Paris climate accord – and said the impacts on humanity could be "long, slow, lingering and horrible".