Global investment in energy fell by 8% last year to $1.8tn (£1.4), reflecting low oil and gas prices and cost falls in the sector, new data shows. Nearly half of the decline was accounted for by the US, where plunging oil prices and a recent boom in shale gas, along with cost deflation in the energy sector, have played an increasing role. China remained the world’s biggest investor in energy worldwide, with $315bn spent in…