There are whales alive in the Arctic today that were born before Moby-Dick was published in 1851. Despite this extraordinary fact, humans kill about 300,000 whales, dolphins and porpoises every year – most die indiscriminately when they get caught up in fishing gear.

Our oceans and the life they sustain are under mounting pressure from multiple threats, including overfishing, climate breakdown, oil drilling and plastic pollution. Quite simply, they are in crisis.

As a passionate advocate for marine conservation, last year I was one of almost three million people who backed Greenpeace’s Protect the Antarctic campaign, calling for the creation of a 1.8m sq km (695,000 sq mile) ocean sanctuary. The proposed sanctuary in the Weddell Sea would have been the largest protected area on Earth, offering a safe haven for penguins, seals, whales and other marine life. It would have made these pristine waters off-limits to oil exploration and the fishing industry.

Sadly, this historic opportunity was lost when members of the Antarctic Ocean commission from China, Norway and Russia derailed the process and blocked the proposal.

But the fight did not end there. Today, governments will begin negotiating the first draft text of a global ocean treaty at the UN in New York that would cover waters that lie beyond national borders. This vast expanse of sea covers almost 50% of the Earth’s surface. If they get it wrong the treaty could entrench many of the worst practices that are impacting our oceans. But if they get it right the treaty could pave the way for the creation of a network of ocean sanctuaries, making 30% of our marine world off-limits to human activity.

The science is clear. Oceans are warming and becoming more acidic, which is killing coral reefs and other fragile ecosystems. Plastic pollution is choking marine life and 90% of large fish such as sharks, swordfish and tuna have been hunted from our seas. The lack of effective governance in international waters has left them open to exploitation from fisheries and extractive industries such as oil and gas. Now a new threat is emerging. Leading scientists have warned that our oceans face severe and irreversible harm from deep-sea mining, with companies queuing up to extract metals and minerals from the seabed.

Paradoxically, while the UK government has championed marine protection, it is also leading the rush to exploit the deep seas. The UK holds licences to explore larger areas of the seabed than any other nation with the exception of China. It seems that the environmental havoc caused by the oil and gas industry has taught us little. Barely 0.01% of these international waters have been explored or studied and yet the pursuit of profit is being placed before environmental protection and scientific research.

‘Leading scientists have warned that our oceans face severe and irreversible harm from deep-sea mining, with companies queuing up to extract metals and minerals from the seabed.’ Photograph: Nautilus minerals

What we do know is that life on Earth depends on healthy oceans. Our fate and the fate of our seas are intimately and inextricably connected. Ocean sanctuaries would provide protection for wildlife populations and ecosystems, allowing them to recover and thrive. The wider benefits would be felt in other ocean areas that would continue to provide food security and livelihoods for millions of people. Healthy oceans also play a crucial role in slowing climate breakdown. The myriad creatures and plants in our seas capture and store carbon from the atmosphere, locking it away underwater. Damaged and depleted oceans will only worsen the climate emergency we are facing.

We need a strong global ocean treaty because right now there is no single regulatory body that bears overall responsibility for effectively managing and safeguarding our oceans. Only 0.8% of international waters are effectively protected. Ocean governance is fragmented across various bodies, leaving marine life vulnerable to ever-expanding human activities.

We have the solutions. We must tackle the root causes of the climate emergency, stop burning fossil fuels and transition to 100% renewable energy. And we must also look after our oceans to mitigate the impacts of a warming climate. Greenpeace and other environmental organisations, together with scientists and a growing number of governments, are calling for a network of ocean sanctuaries covering at least a third of the world’s oceans by 2030. A global ocean treaty is the first step towards ensuring that our blue planet will continue to sustain the lives of billions of people.

Governments must seize this opportunity and write a new chapter for ocean governance that places conservation and sustainability at its centre. Now is the time to match ambition with action, to set aside geopolitical differences and commercial gain and act in the interests of future generations. We have a historic opportunity to strengthen ocean protection for decades to come. What happens in New York this month will have profound consequences for the future of humanity. This is a defining moment for the UK government to step up and show global leadership. Millions of us are watching, and hopefully it will do just that.

• Gillian Anderson is an actor and activist