A group of more than 200 scientists will on Monday urge returning parliamentarians to urgently reduce Australia’s total greenhouse gas emissions, and work diplomatically to achieve coordinated global climate action, after a catastrophic summer of fires.

In an open letter timed to coincide with the resumption of the parliamentary year in Canberra, the group says scientific evidence unequivocally links human-caused climate change to the increasing risk of frequent and severe bushfires in the Australian landscape.

It says that same science tells us “these extreme events will only grow worse in the future without genuine concerted action to reduce global emissions of greenhouse gases”. The letter says the science suggests a need for immediate action to reduce total greenhouse gas emissions, and manage a rapid transition to net zero emissions by 2050.

One of the signatories to the open letter, the Australian National University climate scientist Nerilie Abram, says the letter is “the product of despair as scientists witnessed the deadly fire season unfold”.

“Scientists have been warning policymakers for decades that climate change would worsen Australia’s fire risk, and yet those warnings have been ignored,” she said.

Given the catastrophic summer, with the bushfires triggering a barrage of commentary about the inadequacy of the Morrison government’s current policy, a number of Liberal MPs are returning to Canberra for the opening session parliament with the view that the government needs to do more on climate change and energy policy. Cabinet has also discussed the issue.

The strongly worded appeal by scientists comes as a new energy market analysis by Reputex predicts Australia will hit 50% renewable electricity by 2030, despite the lack of a federal energy policy. But it also warns a slump in new investment in wind and solar investment could threaten a continued decline in wholesale electricity prices.

Reputex says the current drive to 50% renewables by 2030 is being driven by state renewable energy targets and rooftop solar schemes that are predicted to make higher cost gas and coal-fired power less competitive. It says that transition is expected to deliver a decline in wholesale prices in the national energy market from around $80 per megawatt-hour (/MWh) in 2020 toward $70/MWh over the next three years, which translates as a fall of 15% from today’s levels.

But it warns investment in renewable energy plunged by 50% last year compared with the year before, and it predicts that decline will continue – a development that will put upward pressure on wholesale electricity prices, particularly as major coal-fired facilities begin to close.

“In the absence of an effective policy framework to guide new investment, the decline of our ageing generation fleet will lead to higher electricity prices before the new supply is developed, hurting both businesses and consumers,” the assessment says.

In an effort to emphasise a positive message on the issue, the treasurer, Josh Frydenberg, told the ABC on Sunday the Morrison government’s 2030 emissions reduction target of 26% on 2005 levels by 2030 was a floor and not a cap, “and we hope to beat our target”.

But while declaring the government wanted to beat the 2030 target, Frydenberg said the target wasn’t going to be adjusted.

“We took to the Australian people a very clear target, so we’re not about to lift that target,” the treasurer said. “What we are endeavouring to do is to meet our commitments now.”

Frydenberg did not mention that the government intends to use an accounting measure, carryover credits from the Kyoto period, to meet the 2030 target, with carryover credits, not practical abatement, supplying about half the pollution reduction load to 2030.

The treasurer also played up the government’s commitment to renewable energy. Frydenberg said on Sunday that more than $7bn was invested in renewables last year. He omitted the Coalition’s efforts under Tony Abbott to gut the renewable energy target (RET), and the fact the RET winds down from this year – with policy uncertainty triggering the decline in new investment captured in the Reputex analysis.

Asked whether he believed there was now a climate emergency, the treasurer hedged. “Climate change is a significant challenge, a global challenge that needs a global solution,” he said.

“We’re doing our bit and we’re also working on the international stage.”

Pressed to explain how climate change was not an emergency, Frydenberg said “it’s an important issue, but as the prime minister has outlined, there are a lot of things that we can do with mitigation and adaptation to try to reduce the impact of climate change on the Australian community”.