The rainy season has been starting earlier and earlier every year due to abnormal weather caused by global warming.
この場合、次のような言い方ができますよ。
ーThe rainy season has been starting earlier and earlier every year due to abnormal weather caused by global warming.
「地球温暖化が原因で起こる異常気象のため、雨季が年々早まっている」
ここの「早まっている」は、... has been starting earlier and earlier を使って表現すると良いでしょう。
rainy season「雨季」
abnormal weather「異常気象」
global warming「地球温暖化」
ご参考まで!