Complex sentence: From 2013, Stoltenberg served as a UN special envoy on climate change (global warming), and has previously chaired the UN High-Level Panel on System Wide Coherence and the High-Level Advisory Group on Climate Change Financing.
Split into simpler sentences: From 2013, Stoltenberg served as a United Nations (UN) special envoy on climate change (global warming). He has chaired the UN High-Level Panel on System Wide Coherence and the High-Level Advisory Group on Climate Change Financing.