The government needs to revise its estimates of sea level rises to account for increased carbon dioxide emissions, according to Philip Dyke from the National Trust.
Mr Dyke was giving evidence to the Environment and Sustainability Committee on its inquiry into coastal protection in Wales.
Whilst he complimented the government on its national coastline strategy by calling it a "great document", Mr Dyke voiced his concerns over what he felt were conservative estimates of how much carbon dioxide is being released into the atmosphere.
He said: "We are not a medium emission world; we are a high emission world. We leave ourselves hostage to fortune."
Mr Dyke added that current estimates of sea levels rises should be revised in order to take this into account.
The National Trust's written evidence emphasises the need to move beyond a 'defend or do nothing approach' to coastal defence schemes, whilst Mr Dyke was also keen to emphasise the need to raise public awareness of coastline management.
Greg Guthrie from Royal Haskoning warned that the issue of coastal erosion tends to be "underestimated".
The government's national strategy for flood and coastal erosion risk management recognises this.
According to the government, current estimates show that one in six properties in Wales are at risk of flooding from rivers, the sea and surface water.
Cardiff South and Penarth AM Vaughan Gething said that there are currently 12,000 homes at risk from coastal erosion.
AMs heard evidence that one of the main problems in implementing shoreline management plans (SMPs) is that the timescales involved are often very long.
Kath Winnard, from engineering firm Atkins, said: "50 to 100 years are what we talk about in terms of coastal erosions, but local authorities deal with timelines closer to ten years."
Ms Winnard added: "They're not thinking beyond their own horizon."
Ms Winnard also said that the future of flood defences in Borth, where floods hit
, needed to be considered as well at the Cambrian Railway all the way up to Pwllheli.
According to Ms Winnard: "Borth people know these defences won't be there forever."
The committee also heard evidence that communication with the public needed to be improved in order for the national strategy to be effective.
Greg Guthrie said: "At the moment we are behind the gain line, we have applications coming in from people who don't understand the issues."
Mr Guthrie added: "We need to get across the message that the way we manage the coast is vitally important to Wales."
Read this in Welsh.